Section 7 of Gray's Anatomy Part 4, this is LibriVox Recording. All LibriVox recordings are in the public domain. For more information of the volunteer, please visit LibriVox.org. Recording by ML Cohen. Anatomy of the Human Body Part 4 by Henry Gray. The Spinal Cord Part 3. fasciculi in the lateral funiculus. 1. Descending fasciculi. a. The lateral cerebrospinal fasciculus. fasciculi cerebrospinal lateralis. cross-paramidal tract. extends throughout the entire length and medullus spinalis. and, on transverse section, appears as an oval area in front of the posterior column and medial to the cerebellum spinal. Its fibers arise from the cells in the motor area of the cerebral hemisphere of the opposite side. They pass downward, in company with that of the anterior cerebrospinal fasciculus through the same side of the brain as that from which they originate, but they cross to the opposite side in the medulla oblongata and descend in the lateral funiculus of the medullus spinalis. It is probable that the fibers of the anterior and lateral cortical spinal fasciculus are not related in this direct manner with the cells of the anterior column, but terminate by arborizing around the cells at the base of the posterior column and the cells of Clark's column, which link them to the motor cells in the anterior column, usually of several segments of the cord. In consequence of these interposed neurons, the fibers of the cerebral spinal fasciculi correspond not to individual muscles, but to associated groups of muscles. The anterior and lateral cerebral spinal fasciculi constitute the motor fasciculi of the medulla spinalis and have their origins in the motor cells of the cerebral cortex. They descend through the internal capsule of the cerebrum, traverse the cerebral peduncles and pons and enter the pyramid of the medulla oblongata. In the lower part of the ladder, about two-thirds of them cross in the middle line and run downward in the lateral funiculus as the lateral cerebral spinal fasciculus, while the remaining fibers do not cross in the middle line but are continued into the same side in the medulla spinalis where they form the anterior cerebral spinal fasciculus. The fibers of the ladder, however, cross the midline in the anterior white commissure and thus all the motor fibers from one side of the brain ultimately reach the opposite side of the medulla spinalis. The proportion of fibers which cross in the medulla oblongata is not a constant one, and thus the anterior and lateral cerebral spinal fasciculi vary inversely in size. Sometimes the former is absent and in such cases it may be presumed that the decassation in the motor fibers in the medulla oblongata has been complete. The fibers in these two fasciculi do not acquire their medullary seas until after birth. In some animals, the motor fibers are situated in the posterior funiculus. B. The rubro spinal fasciculus, monocow, pre-parametal tract, lies on the ventral aspect of the lateral cerebral spinal fasciculus and on transverse section appears a somewhat triangular area. Its fibers descend from the midbrain where they have their origin in the red nucleus of the tegmentum of the opposite side. Its terminals and collaterals end either directly or indirectly in relationship with the motor cells of the anterior column. The rubro spinal fasciculus is supposed to be concerned with cerebellar reflexes since its afferent fibers which pass through the superior cerebellar peduncle send many collaterals and terminals to the red nucleus. C. The olivospinal fasciculus, helwig, arises in the vicinity of the inferior olivary nucleus in the medulla oblongata and is seen only in the cervical region of the medulla spinalis where it forms a small triangular area at the periphery close to the most lateral of the anterior nerve roots. Its exact origin and its motive ending have not yet been definitely made out. Two. Ascending fasciculi. A. The dorsal spinal cerebellar fasciculus. Fasciculus cerebellus spinalis, direct cerebellar tract of fleishig, is situated at the periphery of the posterior part of the lateral finiculus and on transverse section appears as a flattened band reaching as far forward as a line drawn transversely through the central canal. Medially, it is in contact with the lateral cerebrospinal fasciculus, behind with the fasciculus of lisauer. It begins about the level of the second or third lumbar nerve and increasing in size as it ascends passes to the vermis of the cerebellum through the inferior peduncle. Its fibers are generally regarded as being formed by the axons of the cells of the dorsal nucleus, Clark's column. They receive their medullary seeds about the sixth or seventh month of fetal life. Its fibers are supposed to conduct impulses of unconscious muscles since. The superficial anterior lateral fasciculus, tract of gowers, consists of four fasciculi, the ventral spinocerebellar, the lateral spinothalamic, the spinotectal, and the ventral spinothalamic. B. The ventral spinocerebellar fasciculus, gowers, skirts the periphery of the lateral finiculus in front of the dorsal spinocerebellar fasciculus. In transverse section, it is shaped somewhat like a comma, the expanded end of which lies in front of the dorsal spinocerebellar fasciculus, while the tail reaches forward into the anterior finiculus. Its fibers come from the same, but mostly from the opposite side of the medulla, spinalis, and cross both in the anterior white commissure and in the gray commissure. They are probably derived from the cells of the dorsal nucleus and from other cells in the posterior column and the intermediate portion of the gray matter. The ventral spinocerebellar fasciculus begins about the level of the third pair of lumbar nerves and can be followed into the medulla oblongata and pons almost to the level of the inferior colliculus, where it crosses over the superior peduncle and then passes backward along its medial border to reach the verminus of the cerebellum. In the pons, it lies along the lateral edge of the lateral lamniscus. Some of its fibers join the dorsal spinocerebellar fasciculus at the level of the inferior peduncle and pass with them into the cerebellum. Other fibers are said to continue upward in the dorsal lateral part of the tegmentum of the midbrain, probably as far as the thalamus. C. The lateral spinothalamic fasciculus is supposed to come from cells in the dorsal column and the intermediate gray matter whose axons cross in the anterior commissure to the opposite lateral finiculus where they pass upward on the medial side of the ventral spinocerebellar fasciculus. On reaching the medulla oblongata, they continue in the formacea reticularis near the median filet and probably terminate in the ventral lateral region of the thalamus. It is supposed to conduct impulses of pain and temperature. The lateral and ventral spinothalamic fasciculi are sometimes termed the secondary sensory fasciculus or spinal lamniscus. D. The spinotectal fasciculus is supposed to arise in the dorsal column and terminate in the possibly inferior and superior colliculi. It is situated ventral to the lateral spinothalamic fasciculus, but its fibers are more or less intermingled with it. It is also known as the spinal quadrageminal system of mat. In the brainstem, the fibers run lateral from the inferior olive, ventral lateral from the superior olives, then ventromedial from the spinal tract of the trigeminal. The fibers come to lie in the medial portion of the lateral lamniscus. E. The fasciculus of lisaur is a small strand situated in relation to the tip of the posterior column close to the entrance of the posterior nerve roots. It consists of fine fibers which do not receive their medullary sheaths until towards the close of fetal life. It is usually regarded as being formed by some of the fibers of the posterior nerve roots, which ascend for a short distance in the tract and then enter the posterior column. But since its fibers are myelinated later than those of the posterior nerve roots and do not undergo degeneration in local motor ataxia, they are probably intersegmental in character. In addition, the fasciculus or tract of lisaur contains great numbers of fine, non-measulated fibers derived mostly from the dorsal roots but partly endogenous in origin. These fibers are intimately related to the substantia gelatinosa, which is probably the terminal nucleus. The non-measulated fibers ascend or descend for short distances not exceeding one or two segments, but most of them enter the substantia gelatinosa at or near the level of their origin. Ransom suggests that these non-measulated fibers and the substantia gelatinosa are concerned with the reflexes associated with pain impulses. F. The lateral proper fasciculus, fasciculus lateralis proprious, lateral basis bundle, constitutes the remainder of the lateral column and is continuous in front with the anterior proper fasciculus. It consists chiefly of intersegmental fibers which arise from cells in the gray substance and, after a longer or shorter course, re-enter the gray substance and ramify in it. Some of its fibers are, however, continue to upward into the brain in the name of the medial longitudinal fasciculus, fasciculi in the posterior finiculus. This finiculus comprises two main fasciculi, that is, the fasciculus gracilis and the fasciculus cuneatus. These are separated from each other in the cervical and upper thoracic regions by the posterior intermediate septum and consist mostly of ascending fibers derived from posterior nerve roots. The fasciculus gracilis, which consists of gall, is wedge-shaped on transverse section and lies next to the posterior median septum, its base being at the surface of the medulla spinalis and its apex directed towards the posterior gray commissure. It increases in size from below upward and consists of long, thin fibers which are derived from the posterior nerve roots and ascend as far as the medulla oblongata where they end in the nucleus gracilis. The fasciculus cuneatus, which is a triangular on transverse section and lies between the fasciculus gracilis and the posterior column. Its base corresponding with the surface of the medulla spinalis. Its fibers, larger than those of the fasciculus gracilis, are mostly derived from the same source, that is, the posterior nerve roots. Some may send only for a short distance in the tract and, entering the gray matter, come in close relationship with the cells of the dorsal nucleus while others can be traced as far as the medulla oblongata where they end in the gracil and cuneate nuclei. The fasciculus gracilis and fasciculus cuneatus conduct, one, impulses of conscious muscle sense. Neurons of the second order from the nucleus gracilis and nucleus cuneatus pass an immediate alumniscus to the thalamus and neurons of the third order from the thalamus to the cerebral cortex. Two, impulses of unconscious muscle sense via neurons of the second order from the nucleus gracilis and nucleus cuneatus pass in the internal and external arcuate fibers in the medulla oblongata to the inferior peduncle and through it to the cerebellum. Three, impulses of tactile discrimination via neurons of the second order from the nucleus cuneatus and nucleus gracilis pass in the median alumniscus to the thalamus neurons of the third order pass from the thalamus to the cortex. The posterior proper fasciculus, posterior grand bundle, posterior basis bundle arises from cells in the posterior column. Their axons bifurcate into ascending and descending branches, which occupy the ventral part of the finiculus close to the gray column. They are intersegmental and run for varying distances sending off collaterals and terminals to the gray matter. Some descending fibers occupy different parts at different levels. For collon upper thoracic regions, they appear as a comma-shaped fasciculus in the lateral part of the fasciculus cuneatus, the blunt end of the comma being directed towards the posterior gray commissure. In the lower thoracic region, they form a dorsal peripheral band on the posterior surface of the finiculus. In the lumbar region, they are situated by the side of the posterior median septum and appear on section as a semi-elliptical bundle which together with the corresponding bundle of the opposite side forms the oval area of flyshig. While in the conus medularis, they assume the form of a triangular strand in the posterior medial part of the fasciculus gracilis. These descending fibers are mainly intersegmental in character and derived from the cells in the posterior columns, but some consist of the descending branches of posterior nerve roots. The comma-shaped fasciculus was supposed to belong to the second category, but against this view there is a fact that it does not undergo descending degeneration when the posterior nerve roots are destroyed. As already stated, each spinal nerve possesses two roots, an anterior and a posterior, which are attached to the surface of the medulla spinalis opposite the corresponding column of gray substance. Their fibers become medulated about the fifth month of fetal life. The anterior nuvor, radix anterior, consists of efferent fibers which are the axons of the nerve cells in the ventral portion of the anterior and lateral columns. A short distance from their origins, these axons are invested by medullary seeds and, passing forward, emerge in two or three irregular rows over an area which measures about three millimeters in width. The posterior root, radix posterior, comprises some six or eight fasciculi attached in linear series along the posterior lateral sulcus. It consists of efferent fibers which arise from the nerve cells in the spinal ganglion. Each ganglion cell gives off a single fiber which divides in a T-shaped manner into two processes, medial and lateral. The lateral processes extend to the sensory end organs of the skin, muscle, tendons, joints, etc., somatic receptors, and to the sensory end organs of the viscera, visceral receptors. The medial processes of the ganglion cells grow into the medulla spinalis as the posterior roots of the spinal nerves. The posterior nerve root enters the medulla spinalis in three chief bundles, medial, intermediate, and lateral. The medial strand passes directly into fasciculus cuneatus. It consists of coarse fibers which acquire their medulla received about the fifth month of intrauterine life. The intermediate strand consists of coarse fibers which enter the latinus substance of Rolando. The lateral is composed of fine fibers which assume a longitudinal direction in the tract of Lysauer and do not acquire their medullaries until after birth. In addition to these medullated fibers there are great numbers of non-medullated fibers which enter with the lateral bundle. They are more numerous than the myelinated fibers. They arise from the small cells of the spinal ganglia by T-shaped axons similar to the myelinated. They are distributed with the peripheral nerves chiefly to the skin and only a few are found in the nerves to the muscles. Having entered the medulla spinalis all the fibers of the posterior nerve roots divide to ascending and descending branches and these in their turn give off collaterals which enter the gray substance. The descending fibers are short and soon enter the gray substance. The ascending fibers are grouped into long, short, and intermediate. The long fibers ascend in the fasciculus mediatus and fasciculus gracilis as far as the medulla oblongata where they end by arborizing around cells of the cuneate and gracil nuclei. The short fibers run upward for a distance of only five or six millimeters and enter the gray substance while the intermediate fibers after a somewhat longer course have a similar destination. All fibers entering the gray substance end by arborizing around its nerve cells or the dendrites of cells. Those of intermediate length being especially associated with the cells of the dorsal nucleus. The long fibers of the posterior nerve roots pursue an oblique course upward being situated at first in the lateral part of the fasciculus cuneatus. Higher up, they occupy the middle of this fasciculus having been displaced by the accession of other entering fibers. While still higher, they ascend in the fasciculus gracilis. The upper cervical fibers do not reach this fasciculus but are entirely confined to the fasciculus cuneatus. The localization of these fibers is very precise. The sacral nerves lie in the medial part of the fasciculus gracilis and near its periphery. The lumbar nerves, lateral to them, the thoracic nerves still more laterally, while the cervical nerves are confined to the fasciculus cuneatus. End of Section 7 Section 8 of Gray's Anatomy Part 4. This is a LibriVox recording. All LibriVox recordings to public domain. For more information or to volunteer, please visit LibriVox.org. Recording by M. L. Cohen. Anatomy of the Human Body, Part 4 by Henry Gray. The hind brain, Part 1. The encephalon or brain. General considerations and divisions. The brain is contained within the cranium and constitutes the upper, greatly expanded part of the central nervous system. In its early embryonic condition, it consists of three hollow vesicles termed the hind brain or rhombencephalon, the midbrain or mesencephalon, and the forebrain or prosencephalon. And the parts derived from each of these can be recognized in the adult. Thus, in the process of development, the wall of the hind brain undergoes modification to form the medulla oblongata, the pons and cerebellum, while its cavity is expanded to form the fourth ventricle. The midbrain forms only a small part of the adult brain. Its cavity becomes the cerebral aqueduct, aqueduct of sylvius, which serves as a tubular communication between the third and fourth ventricles, while its walls are thickened to form the corpora quadragemina and cerebral peduncles. The forebrain undergoes great modifications. Its anterior part or telencephalon expands laterally in the form of two hollow vesicles, the cavities of which become the lateral ventricles, while the surrounding walls form the cerebral hemispheres and their commissures. The cavity of the posterior part, or diencephalon, forms the greater part of the third ventricle, and from its walls are developed most of the structures which bound that cavity. The hindbrain or ramencephalon. The hindbrain or ramencephalon occupies the posterior fossa of the cranial cavity and lies below a fold of duromata than tentorium cerebelli. It consists of a. the myelin cephalon, comprising the medulla oblongata and the lower part of the fourth ventricle, b. the metincephalon, consisting of the pons, cerebellum, and the intermediate part of the fourth ventricle, and c. the isthmus rambencephalae, a constricted portion immediately adjoining the midbrain and including the superior peduncles of the cerebellum, the anterior medullary vellum, and the upper part of the fourth ventricle. The medulla oblongata, spinal bulb. The medulla oblongata extends from the lower margin of the pons to a plane passing transversely below the pyramidal decassation and above the first pair of cervical nerves. This plane corresponds with the upper border of the atlas behind and the middle of the odontoid process of the axis in front. At this level the medulla oblongata is continuous with the medulla spinalis. Its anterior surface is separated from the basilar part of the occipital bone and the upper part of the odontoid process by the membranes of the brain and the occipital axial ligaments. Its posterior surface is received into the fossa between the hemispheres of the cerebellum and the upper portion of it forms the lower part of the floor of the fourth ventricle. The medulla oblongata is pyramidal in shape. Its broad extremity being directed upwards towards the pons while its narrow lower end is continuous with the medulla spinalis. It measures about 3 cm in length, about 2 cm in breadth at its widest part and about 1.25 cm in thickness. The central canal of the medulla spinalis is prolonged into its lower half and then opens into the cavity of the fourth ventricle. The medulla oblongata may therefore be divided into a lower closed part containing the central canal and an upper open part corresponding with the lower portion of the fourth ventricle. The anterior median fissure fissura mediana anterior ventral or ventromedian fissure contains a fold of pia mater and extends along the entire length of the medulla oblongata. It ends at the lower border of the pons in a small triangular expansion termed the foramen cecum. Its lower part is interrupted by the folds of fibers which cross obliquely from one side to the other and constitute the pyramidal decassation. Some fibers, termed the anterior external arcuate fibers, emerge from the fissure above this decassation and curve lateralward and upward over the surface of the medulla oblongata to join the inferior peduncle. The posterior median fissure fissura mediana posterior dorsal or dorsal medial fissure is a narrow groove and exists only in the middle of the medulla oblongata. It becomes gradually shower from below upward and finally ends about the middle of the medulla oblongata where the central canal expands into the cavity of the fourth ventricle. These two fissures divide the close part of the medulla oblongata into symmetrical halves each presenting elongated eminences which on the surface view are continuous with the funiculi of the medulla spinalis. In the open part the halves are separated by the anterior median fissure and by a median raffae which extends from the bottom of the fissure to the floor of the fourth ventricle. Further, certain of the cranial nerves pass through the substance of the medulla oblongata and are attached to its surface and series with the roots of the spinal nerves. Thus the fibers of the hypoglossal nerve represent the upward continuation of the anterior nerve roots and emerge in linear series from a furrow termed the anterior lateral sulcus. Similarly the accessory vagus and glossopharyngeal nerves correspond with the posterior nerve roots and are attached to the bottom of a sulcus named the posterior lateral sulcus. Advantage is taken of this arrangement to subdivide each half of the medulla oblongata into three districts anterior, middle and posterior. Although these three districts appear to be directly continuous with the corresponding funiculi of the medulla spinalis they do not necessarily contain the same fibers since some of the funiculi of the medulla spinalis end in the medulla oblongata while others alter their course in passing through it. The anterior district is named the pyramid Pyramus medulla oblongata and lies between the anterior median fissure and the anterior lateral sulcus. Its upper end is slightly constricted and between it and the ponds the fibers of the abducent nerve emerge. A little below the ponds it becomes enlarged and prominent finally tapers into the anterior funiculus of the medulla spinalis with which, at first sight, it appears to be directly continuous. The two pyramids contain the motor fibers which pass from the brain to the medulla oblongata and medulla spinalis cortical bulbar and cortical spinal fibers. When these pyramidal fibers are traced downward it is found that some two-thirds or more of them leave the pyramids in successive bundles and decasate in the anterior median fissure forming what is termed the pyramidal decasation. Having crossed the middle line they pass down in the posterior part of the lateral funiculus as the lateral cerebrospinal fissiculus. The remaining fibers, that is, those which occupy the lateral part of the pyramid do not cross the middle line but are carried downward as the anterior cerebrospinal fissiculus into the anterior funiculus of the same side. The greater part of the anterior proper fissiculus of the medulla spinalis is continued upward through the medulla oblongata under the name of the medial longitudinal fissiculus. The lateral district is limited in front by the anterior lateral sulcus and the roots of the hypoglossal nerve and behind by the posterior lateral sulcus and the roots of the accessory, vagus and glossopharyngeal nerves. Its upper part consists of a prominent oval mass which is named the olive while its lower part is of the same width as the lateral funiculus of the medulla spinalis and goes on the surface to be a direct continuation of it. As a matter of fact, only a portion of the lateral funiculus is continued upward into this district. For the lateral cerebrospinal fissiculus, passes into the pyramid of the opposite side and the dorsal spinocerebellar fissiculus is carried into the inferior peduncle in the posterior district. The ventral spinocerebellar fissiculus is continued upward on the lateral surface of the medulla oblongata in the same relative position while it passes under cover of the external arcuate fibers. It passes beneath these fibers just dorsal to the olive and ventral to the roots of the vagus and glossopharyngeal nerves. It continues upward through the pons along the dorsal lateral edge of the lateral amniscus. The remainder of the lateral funiculus consists chiefly of the lateral proper fissiculus. Most of these fibers dip beneath the olive and disappear from the surface but a small strand remains superficial to the olive. In a depression at the upper end of this strand is the acoustic nerve. The olive oliva, olivary body is situated lateral to the pyramid from which it is separated by the anterior lateral sulcus and the fibers of the hypoglossal nerve. Behind, it is separated from the posterior lateral sulcus by the ventral spinocerebellar fissiculus. In the depression between the upper end of the olive and the pons lies the acoustic nerve. It measures about 1.25 centimeters in length and between its upper end and the pons there is a slight depression to which the roots of the facial nerve are attached. The external arcuate fibers wind across the lower part of the pyramid and olive and enter the inferior peduncle. The posterior district lies behind the posterior lateral sulcus and the roots of the accessory vagus and glossopharyngeal nerves and, like the lateral district, is divisible into a lower and an upper portion. The lower part is limited behind by the posterior median fissure and consists of the fissiculus gracilis and the fissiculus cuneatus. The fissiculus gracilis is placed parallel to and along the side of the posterior median fissure and separated from the fissiculus cuneatus by the posterior intermediate sulcus and septum. The gracil and cuneate fissiculi are at first vertical in direction but at the lower part of the rhomboid fossa they diverge from the middle line in a V-shaped manner and each presents an elongated swelling. That on the fissiculus gracilis is named the clava and is produced by its subjacent nucleus of grey matter, the nucleus gracilis. That on the fissiculus cuneatus is termed the cuneate tubercle and is likewise caused by a grey nucleus named the nucleus cuneatus. The fibers of these fissiculi terminate by arborizing around the cells in the respective nuclei. The third elevation produced by the substantial gelatinosis of Rolando is present in the lower part of the posterior district of the medulla oblongata. It lies on the lateral aspect of the fissiculus cuneatus and is separated from the surface of the medulla oblongata by a band of nerve fibers which form the spinal tract, spinal root of the trigeminal nerve. Narrow below this elevation gradually expands above and ends about 1.25 centimeters below the pons in a tubercle. The tubercle of Rolando, tuberosinarium. The upper part of the posterior district of the medulla oblongata is occupied by the inferior peduncle, a thick rope-like strand situated between the lower part of the fourth ventricle and the roots of the glasopharyngeal and vagus nerves. The inferior peduncles connect the medulla spinalis and medulla oblongata with the cerebellum and are sometimes named the rest-to-form bodies. As they pass upward, they diverge from each other by the persistent forming the lower part of the lateral boundaries of the fourth ventricle. Higher up, they are directed backwards, each passing to the corresponding cerebellar hemisphere. Near their entrance into the cerebellum, they are crossed by several strands of fibers which run to the median sulcus of the rhomboid fossa and are named the stria medullaries. The inferior peduncle appears to be the upward continuation of the fissiculus gracilis and fissiculus cuneatus. This, however, is not so. The fissiculi end in the gracilin cuneate nuclei. The constitution of the inferior peduncle will be subsequently discussed. Caught up to the stria medullaries, the inferior peduncle is partly covered by the corpus pontobulbar, a thin mass of cells and fibers extending from the ponds between the origin of the seventh and eighth cranial nerves. Internal structure of the medulla oblongata. Although the external form of the medulla oblongata differs a certain resemblance to the upper part of the medulla spinalis, its internal structure differs widely from that of the latter and this for the following principal reasons. One, certain fissiculi which extend from the medulla spinalis to the brain and vice versa undergo arrangement in their passage through the medulla oblongata. Two, others which exist in the medulla spinalis end in the medulla oblongata. Three, new fissiculi originate in the gray substance of the medulla oblongata and pass to different parts of the brain. Four, the gray substance which in the medulla spinalis forms a continuous eight shaped column becomes greatly modified and subdivided in the medulla oblongata where also new masses of gray substance are added. Five, on account of the opening out of the central canal of the medulla spinalis, certain parts of the gray substance which in the medulla spinalis were more or less centrally situated are displaced in the rhomboid fossa. Six, the medulla oblongata is intimately associated with many of the cranial nerves summarizing from and others ending in nuclei within its substance. The cerebrospinal fissiculi. The downward course of these fissiculi from the pyramids of the medulla oblongata and their partial decusation have already been described. In crossing to reach the lateral finiculus of the opposite side the fibers of the lateral cerebrospinal fissiculi extend backwards through the anterior columns and separate the head of each of these columns from its base. The base retains its position in relation to the ventral aspect of the central canal and when the ladder opens into the fourth ventricle appears in the rhomboid fossa close to the middle line where it forms the nuclei of the hypoglossal and abducent nerves. While above the level of the ventricle it exists as the nuclei of the trochlear and oculomotor nerves in relation to the floor of the cerebral aqueduct. The head of the column is pushed lateral word and forms the nucleus ambiguous which give origin from below upward to the cranial part of the accessory and the motor fibers of the vagus and glossopharyngeal and still higher to the motor fibers of the facial and trigeminal nerves. The fissiculus gracilis and fissiculus cuneatus constitute the posterior sensory fissiculi of the medulla spinalis. They are prolonged upward into the lower part of the medulla oblongata where they end respectively in the nucleus gracilis and nucleus cuneatus. These two nuclei are continuous with the central gray substance of the medulla spinalis and may be regarded as dorsal projections of this each being covered superficially by the fibers of the corresponding fissiculus. On transverse section the nucleus gracilis appears as a single more or less quadrangular mass while the nucleus cuneatus consists of two parts a larger somewhat triangular medial nucleus composed of smaller medium sized cells and a small lateral nucleus containing large cells. The fibers of the fissiculus gracilis and fissiculus cuneatus end by arborizing around the cells of these nuclei. From the cells of the nuclei new fibers arise. Some of these are continued as the posterior external arcuate fibers into the inferior peduncle and through it to the cerebellum but most of them pass forward through the neck of the posterior column thus cutting off its head from its base. Curving forward they decussate in the middle line with the corresponding fibers of the opposite side and run upward immediately behind the cerebrospinal fibers as a flattened band named the lemniscus or filet. The decussation of these sensory fibers is situated above that of the motor fibers and is named the decussation of the lemniscus or sensory decussation. The lemniscus is joined by the spinal thalamic fissiculus the fibers of which are derived from the cells of the gray substance of the opposite side than medulla spinalis. The base of the posterior column at first lies on the dorsal aspect of the central canal but when the ladder opens into the fourth ventricle it appears in the lateral part of the rhomboid fossa. It forms a terminal nuclei of the sensory fibers of the vagus and glossopharyngeal nerves and is associated with the estipular part of the acoustic nerve and a sensory root of the facial nerve. Still higher it forms a mass of pigmented cells that are ceruleous in which some of the sensory fibers of the trigeminal nerve appear to end. The head of the posterior column forms a long nucleus in which the fibers of the spinal tract of the trigeminal nerve largely end. The dorsal spinocerebellar fissiculus fissiculus cerebellus spinalis direct cerebellar tract leaves the lateral district of the medulla oblongata. Most of its fibers are carried backward into the inferior peduncle of the same side and through it are conveyed to the cerebellum. But some run upward with the fibers of the lamniscus and reaching the inferior colliculus undergo decassation and are carried to the cerebellum through the superior peduncle. The proper fissiculi, basis bundles of the anterior and lateral finiculi largely consist of intersegmental fibers which link together the different segments of the medulla spinalis. They assist in the production of the fomacea particularis in the medulla oblongata and many of them are accumulated into a fissiculus which runs up close to the median raffae between the lamniscus and the rhomboid fossa. This strand is named the medial longitudinal fissiculus and will be again referred to. End of Section 8 Section 9 of Grey's Anatomy Part 4 This is the LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer please visit LibriVox.org Recording by ML Cohen Anatomy of the Human Body Part 4 by Henry Gray The Hindbrain Part 2 Grey substance of the medulla oblongata In addition to the gracil and cuneate nuclei there are several other nuclei to be considered. Some of these are traceable from the grey substance of a medulla spinalis while others are unrepresented in it. 1. The hypoglossal nucleus is derived from the base of the anterior column. In the lower closed part of the medulla oblongata it is situated on the ventrolateral aspect of the central canal but in the upper part it approaches the rhomboid fossa where it lies close to the middle line under an eminence named the trigonome hypoglossi. Numerous fibers connected to nuclei both nuclei send long dendrons across the midline to the opposite nucleus. Commissure fibers also connect them. The nucleus measures about 2 centimeters in length and consists of large multipolar nerve cells similar to those in the anterior column of the spinal cord whose axons constitute the roots of the hypoglossal nerve. These nerve roots leave the ventral side of the nucleus pass forward between the white reticular formation and the grey reticular formation, some between the inferior olivary nucleus and the medial accessory olivary nucleus and emerge from the anterior lateral sulcus. 2. The nucleus ambiguous the somatic motor nucleus of the glossopharyngeal, vagus and cranial portion of the accessory nerves is the continuation into the medulla oblongata of the dorsolateral cell group of the anterior column of the spinal cord. Its large multipolar cells are like those in the anterior column of the cord. They form a slender column in the deep part of the formation reticularis grissier about midway between the dorsal accessory olive and the nucleus of the spinal tract of the trigeminal. It extends from the level of the decassation of the median filet to the upper end of the medulla oblongata. Its fibers first pass backward towards the floor of the fourth ventricle and then curve rather abruptly lateral word and venturally to join the fibers from the dorsal nucleus. The dorsal nucleus, nucleus allocineria often called the sensory nucleus or the trigeminal nucleus or the sensory fibers of the glossopharyngeal and vagus nerves is probably in mixed nucleus and contains not only the terminations of the sympathetic afferent or sensory fibers and the cells connected with them but contains also cells which give rise to the sympathetic efferent or preganglionic fibers. These preganglionic fibers terminate in sympathetic ganglia from which the impulses are carried by other neurons. The cells of the dorsal nucleus are spindle shaped like those of the posterior column of the spinal cord and the nucleus is usually considered as represented the base of the posterior column. It measures about two centimeters in length and in the lower closed part of the medulla oblongata is situated behind the haupelgasal nucleus whereas in the upper open part it lies lateral to that nucleus and corresponds to an eminence named the allocineria trigonum vagae in the rhomboid fossa. 4. The nuclei of the cochlear and vestibular nerves are described later. 5. The olivary nuclei are three in number on either side of the midline that is the inferior olivary nucleus and the medial and dorsal accessory olivary nuclei. They consist of small round yellowish cells and numerous fine nerve fibers. A. The inferior olivary nucleus is the largest and is situated within the olive It consists of a gray-folded lamina arranged in the form of an incomplete capsule open medially by an aperture called the hylum. Emerging from the hylum are numerous fibers which collectively constitute the peduncle of the olive. The axons, olivocerebellar fibers which leave the olivary nucleus pass out through the hylum and decosate with those from the opposite olive in the rathae. Then, as internal arcuate fibers they pass partly through and partly around the opposite olive and the inferior peduncle to be distributed to the cerebellar hemisphere of the opposite side from which they arise. The fibers are smaller than the internal arcuate fibers connected with the medialemniscus. Fibers passing in the opposite direction from the cerebellum to the olivary nucleus are often described, but their existence is doubtful. Much uncertainty also exists in regard to the connections of the olive in the spinal cord. Important connections between the cerebellum to the same side exist, but the exact pathway is unknown. Many collaterals from the reticular formation and from the pyramids enter the inferior olivary nucleus. Removal of one cerebellar hemisphere is followed by atrophy of the opposite olivary nucleus. B. The medial accessory olivary nucleus lies between the inferior olivary nucleus and the pyramid and forms a curved lamina, the concavity of which is directed laterally. The fibers of the hypoglossal nerve, as they traverse the medulla, pass between the medial accessory and the inferior olivary nuclei. C. The dorsal accessory olivary nucleus is the smallest and appears on transfer section as the curved lamina behind the inferior olivary nucleus. Six. The nucleus arcuateus is described below with the anterior external arcuate fibers. Inferior peduncle rest to form body. The position of the inferior peduncles has already been described. Each comprises, fibrous from the dorsal spinocerebellar fasciculus, which ascends from the lateral finiculus of the medulla spinalis. Two. The olivocerebellar fibrous from the opposite olivary nucleus. Three. Internal arcuate fibrous from the gracilin cuneate nuclei of the opposite side. These fibrous form the deeper and larger part of the inferior peduncle. Four. The anterior external arcuate fibrous vary as to the prominence in different cases. In some, they form an almost continuous layer covering the pyramid and olive while in others they are barely visible on the surface. They arise from the cells of the gracilin cuneate nuclei and passing forward through the formacea reticularis decasate in the middle line. Most of them reach the surface by way of the anterior median fissure and arch backward over the pyramid. Reinforced by others which emerge between the pyramid and olive, they pass backwards over the olive and lateral district of the medulla oblongata and enter the inferior peduncle. Thus, they connect the cerebellum with the gracilin cuneate nuclei of the opposite side.  This is named the nucleus arcuateis and is serially continuous above with the nucleus pontis and napons. It contains small fusiform cells around which some of the arcuate fibers end and from which others arise. 5. The posterior external arcuate fibers also take origin in the gracilin cuneate nuclei. They pass to the inferior peduncle of the same side. It is uncertain whether fibers are continued directly from the gracilin cuneate fasciculi into the inferior peduncle. 6. Fibrous from the terminal sensory nuclei of the cranial nerves, especially the vestibular. Some of the fibers of the vestibular nerve are thought to continue directly into the cerebellum. 7. Fibrous from the ventral spinal cerebellar fasciculus. 8. The existence of fibrous from the cerebellum, cerebellal bulbar, cerebellar olivary and cerebellar spinal to the medulla and spinal cord is very uncertain. Formatio reticularis This term is applied to the coarse reticulum which occupies the anterior and lateral districts of the medulla oblongata. It is situated behind the pyramid and olive extending laterally as far as the inferior peduncles and dorsally to within a short distance of the rhomboid fossa. The reticulum is caused by the intersection of bundles of fibers running at right angles to each other, some being longitudinal, others more or less transversed in direction. The formatio reticularis presents a different appearance in the anterior district from what it does in the lateral. In the former there is an almost entire absence of nerve cells and hence this part is known as the reticularis alba. Whereas in the lateral district nerve cells are numerous and as a consequence it presents a gray appearance and is termed the reticularis grizzia. In the substance of the formatio reticularis are two small nuclei of gray matter. One the inferior central nucleus of roller, near the dorsal aspect of the hilus of the inferior olivary nucleus, the other the nucleus lateralis between the olive and the spinal tract of the trigeminal nerve. In the reticularis alba the longitudinal fibers form two well-defined fisculi that is one the lemniscus which lies close to the raffae immediately behind the fibers of the pyramid and two the medial longitudinal fisculus which is continued upward from the anterior and lateral proper fisculi of the medulla spinalis and in the upper part of the medulla oblongata lies between the lemniscus and the gray substance of the rhomboid fossa. The longitudinal fibers in the reticularis grizzia are derived from the lateral finiculus of the medulla spinalis after the lateral cerebrospinal fisculus has passed over to the opposite side and the dorsal spinal cerebellar fisculus has entered the inferior peduncle. They form indeterminate fibers with the exception of a bundle named a fisculus solitarius which is made up of descending fibers of the vagus and glossopharyngeal nerves. The transverse fibers of the formaceo reticularis are the arcuate fibers already described. The pons pons varioli The pons or four part of the hindbrain is situated in front of the cerebellum. From its superior surface the cerebral peduncles emerge one on either side of the middle line. Curving around each peduncle close to the upper surface of the pons a thin white band the tinea pontis is frequently seen. It enters the cerebellum between the middle and superior peduncles. Behind and below the pons is continuous from the medulla oblongata but is separated from it in front by a furrow in which the abducent, facial and acoustic nerves appear. Its ventral or anterior surface paris basilaris pontis is very prominent markedly convex from side to side less so from above downward. It consists of transverse fibers arched like a bridge across the middle line and gathered on either side into a compact mass which forms the middle peduncle. It rests upon the clivus of the spinota bone and is limited above and below by well-defined borders. In the middle line is the sulcus basilaris for the lodgement of the basilar artery. This sulcus is bounded on either side by an eminence caused by the descent of the cerebrospinal fibers through the substance of the pons. Outside these eminences near the upper border of the pons the trigeminal nerves make their exit each consisting of a smaller medromotor root and a larger lateral sensory root. Vertical lines drawn immediately beyond the trigeminal nerves may be taken as the boundaries of the surface of the pons and the middle cerebellar peduncle. Its dorsal or posterior surface paras dorsalis pontis triangular in shape is hidden by the cerebellum and is bounded laterally by the superior peduncle. It forms the upper part of the rhomboid fossa with which it will be described. Structure Transverse sections of the pons show it to be composed of two parts which differ in appearance and structure. Thus, the basilar or ventral portion consists for the most part of fibers arranged in transverse and longitudinal bundles together with a small amount of gray substance. While the dorsal tegmental portion is a continuation of the reticular formation of the medulla oblongata and most of its constituents are continued into the tegmenta of the cerebral peduncles. The basilar part of the pons consists of A. Superficial and deep transverse fibers B. Longitudinal fasciculi and C. Some small nuclei of gray substance term the nuclei pontis which give rise to the transverse fibers. The superficial transverse fibers fibré pontis superficialis constitute a rather thick layer on the ventral surface of the pons and are collected into a large rounded bundle on either side of the middle line. This bundle, with the addition of some transverse fibers from the deeper part of the pons is a greater part of the brachium pontis. The deep transverse fibers fibré pontis profundae partly intersect and partly lie in the dorsal aspect of the cerebrospinal fibers. They course to the lateral border of the pons and form part of the middle peduncle. A further connections of this brachium will be discussed with the anatomy of the cerebellum. The longitudinal fasciculi fasciculi longitudinalis are derived from the cerebropeduncles that are the upper surface of the pons. They stream downward on either side of the middle line in larger or smaller bundles separated from each other by the deep transverse fibers. These longitudinal bundles cause a forward projection of the superficial transverse fibers and thus give rise to the eminences on the anterior surface. Some of these fibers end in or give off collateral to the nuclei pontis. An important pathway is thus formed between the cerebral cortex and the cerebellum which is neuron having its cell body in the cortex and sending its axon through the internal capsule and cerebropeduncle to form synapses either by terminals or collaterals with cell bodies situated in the nuclei pontis. Axons from these cells form the transverse fiber which pass through the middle peduncle into the cerebellum. Others, after decasating, terminate either directly or indirectly in the motor nuclei of the trigeminal, abducent, facial, and hypoglossal nerves carried to the pons and at the lower surface are collected into the pyramids of the medulla. The fibers which end in the motor nuclei of the cranial nerves are derived from the cells of the cerebral cortex and bear the same relation to the motor cells of the cranial nerves that the cerebrospinal fibers bear to the motor cells in the anterior column of the medulla spinalis. Probably none of the collaterals or terminals of the cerebrospinal and cerebral bulbar fibers end directly in the motor nuclei of the spinal and cranial nerves. One or more association neurons are probably interpolated in the pathway. The nuclei pontis are sheerly continuous with the arcuate nuclei in the medulla and consist of small groups of multipolar nerve cells which are scattered between the bundles of the transverse fibers. The dorsal or tegmental part of the pons is chiefly composed of an upward continuation of the reticular formation and gray substance of the medulla oblongata. It consists of transverse and longitudinal fibers and also contains important gray nuclei and is subdivided by a median raffae which however does not extend into the basilar part being obliterated by the transverse fibers. The transverse fibers in the lower part of the pons are collected into a distinct strand named the trapezoid body. This consists of fibers which arise from cells of the cochlear nucleus and will be referred to in connection with the cochlear division of the acoustic nerve. In the substance of the trapezoid body is a collection of nerve cells which constitutes the trapezoid nucleus. The longitudinal fibers which are continuous which those of the medulla oblongata are mostly collected into two fasciculae on each side. One of these lies between the trapezoid body and the reticular formation and forms the upward prolongation of the lemniscus. The second is situated near the floor of the fourth ventricle longitudinal fasciculus. Other longitudinal fibers more diffusely distributed arise from the cells of the grey substance of the pons. The rest of the dorsal part of the pons is a continuation upward of the formacea reticularis of the medulla oblongata and like it presents the appearance of a network in the meshes of which are numerous nerve cells. Besides these scattered nerve cells there are some larger masses of grey substance that is the superior olivary nucleus and the nuclei of the trigeminal adducent facial and acoustic nerves. One. The superior olivary nucleus nucleus olivaris superior is a small mass of grey substance situated on the dorsal surface of the lateral part of the trapezoid body. Rudimentary in man but well developed in certain animals it exhibits the same structure as the inferior olivary nucleus and is situated immediately above it. Some of the fibers of the trapezoid body end by arborizing around the cells of this nucleus while others arise from these cells. Two. The nuclei of the trigeminal nerve. Nuclei and trigeminy. In the pons are two in number a motor and a sensory. The motor nucleus is situated in the upper part of the pons crows to its posterior surface and along the line of the lateral margin of the fourth ventricle. It is serially homologous with the nucleus ambiguous and the dorsal lateral cell group of the anterior column of the spinal cord. The axis cylinder processes of its cells form a portion of the motor root of the trigeminal nerve. The remaining fibers of the motor root of this nerve consist of a fasciculus which arises from the grey substance of the floor of the cerebral aqueduct and hence is named the mesencephalic root. It is not altogether clear whether the mesencephalic root is motor or sensory. The sensory nucleus is lateral and beneath the superior pinoncle. Some of the sensory fibers of the trigeminal nerve end in this nucleus but the greater number descend under the name of the spinal tract of the trigeminal nerve to end in the substantial gelatinosa of Rolando. The roots, motor and sensory of the trigeminal nerve pass through the substance of the pons and emerge near the upper margin of its anterior surface. 3. The nucleus of the abducent nerve is a circular mass of grey substance situated close to the floor of the fourth ventricle, above the stream edularis and subjacent to the medial eminence. It lies a little lateral to the ascending part of the facial nerve. The fibers of the abducent nerve pass forward through the entire thickness of the pons on the medial side of the superior olivary nucleus and between the lateral fasciculi of the cerebrospinal fibers and emerge in the furrow between the lower border of the pons and the pyramid of the medulla oblongata. The nucleus of the facial nerve, nucleus N facialis is situated deeply in the reticular formation of the pons on the dorsal aspect of the superior olivary nucleus and the roots of the nerve derived from it pursue a remarkably tortuous course in the substance of the pons. At first they pass backward of medial word until they reach the rhomboid fossa close to the median sulcus where they are collected into a round bundle. This passes upward and forward producing an elevation, the colliculus facialis in the rhomboid fossa, and then takes a sharp bend and arches lateral word through the substance of the pons to emerge at its lower border in the interval between the olive and the inferior peduncle of the medulla oblongata. Five. The nucleus of the cochlear nerve consists of A, the lateral cochlear nucleus of the lateral surface of the inferior peduncle and B, the ventral or excensury cochlear nucleus placed between the two divisions of the nerve on the ventral aspect of the inferior peduncle. The nuclei of the vestibular nerve A, the medial, dorsal or chief vestibular nucleus corresponding to the lower part of the area acoustica in the rhomboid fossa the caudal end of this nucleus is sometimes termed the descending or spinal vestibular nucleus. B, the lateral or nucleus of deeders consisting of large cells and situated in the lateral angle of the rhomboid fossa the dorsal lateral part of this nucleus is sometimes termed the nucleus of vectoru. The fibers of the vestibular nerve enter the medulla oblongata on the medial side of those of the cochlear and pass between the inferior peduncle and the spinal tract of the trigeminal. They then divide into ascending and descending fibers the latter end by arborizing around the cells of the medial nucleus which is situated in the area acoustica of the rhomboid fossa the ascending fibers either end in the same manner or in the lateral nucleus which is situated lateral to the area acoustica and farther from the ventricular floor some of the axons of the cells of the lateral nucleus and possibly also the medial nucleus are continued upward through the inferior peduncle to the roof nuclei of the opposite side cerebellum to which also other fibers of the vestibular root are prolonged without interruption in the nuclei of the medulla oblongata. A second set of fibers from the medial and lateral nuclei and partly in the tegmentum while the remaining ascend in the medial longitudinal fasciculus to arborize around the cells of the nuclei of the ocular motor nerve. End of section 9 Section 10 of Gray's Anatomy Part 4 this is a LibriVox recording are in the public domain. For more information or to volunteer please visit LibriVox.org Recording by M.L. Cohen Anatomy to Human Body Part 4 by Henry Gray The Hindbrain Part 3 The cerebellum constitutes the largest part of the hindbrain it lies behind the palms of the medulla oblongata between its central portion and these structures is the cavity of the fourth ventricle. The rest on the inferior occipital fosse while above it is the tentorium cerebellae a fold of dura mater which separates it from the tentorial surface of the cerebrum. It is somewhat oval in form but constricted medially and flattened from above downward its greatest diameter being from side to side. Its surface is not convoluted like that of the cerebrum but is traversed by numerous curved furrows or sulci which vary in depth in different parts and separate the laminae of which it is composed. Its average weight in the male is about 150 grams in the adult the proportion between the cerebellum and cerebrum is about 1 to 8 in the infant about 1 to 20 lobes of the cerebellum The cerebellum consists of three parts a median and two lateral which are continuous with each other and are substantially the same in structure. The median portion is constricted and is called the vermis from its annulated appearance which it owes to the transverse ridges and furrows upon it. The lateral expanded portions are named the hemispheres. On the upper surface of the cerebellum the vermis is elevated above the level of the hemisphere but on the under surface it is sunk almost out of sight in the bottom of a deep space between them. This depression is called the velecular cerebellae and lodges the posterior part of the medulla oblongata. The part of the vermis on the upper surface of the cerebellum is named the superior vermis that on the lower surface the inferior vermis. The hemispheres are separated below and behind by a deep notch the posterior cerebellar notch and in front by a broader shallower notch the anterior cerebellar notch. The anterior notch lies close to the pons in upper part of the medulla and its superior edge encircles the inferior caliculi and the superior cerebellar peduncle. The posterior notch contains the upper part of the falc cerebellae, a fold of Dura mater. The cerebellum is characterized by a laminated or foliated appearance. It is marked by deep somewhat curved fissures that are considerable distance into its substance and divided into a series of layers or leaves. The largest and deepest fissure is named the horizontal sulcus. It commences in front of the pons and passes horizontally around the free margin of the hemisphere to the middle line behind and divides the cerebellum into an upper and a lower portion. Several secondary but deep fissures separate the cerebellum into lobes and these are further subdivided by shallower sulci which separate the individual folia or laminae from each other. Sections across the laminae show that the folia though differing in appearance from the convolutions of the cerebrum are analogous to them in as much as they consist of central white substance covered by grey substance. The cerebellum is connected to the cerebrum, pons and medulla oblongata to the cerebrum by the superior peduncle to the pons by the middle peduncle and to the medulla oblongata by the inferior peduncles. The upper surface of the cerebellum is elevated in the middle and sloped towards the circumference the hemisphere is being connected together by a superior vermus which assumes the form of a raised median ridge most prominent in front but not sharply defined from the hemispheres. The superior vermus is subdivided from before backwards into the lingula the lobes centralis the monticulus and the folium vermus and each of these with the exception of the lingula is continuous with the corresponding parts of the hemispheres the lobulocentralis with the aile the monticulus with the quadrageminal lobules and the folium vermus with the superior semilunar lobules the lingula lingula cerebelli is a small tongue shaped process consisting of four or five folia it lies in front of the lobulocentralis and is concealed by it anteriorly it rests on the dorsal surface of the anterior medullary velum and its white substance is continuous with that of the vellum the lobulocentralis and aile the lobulocentralis is a small square lobule situated in the anterior cerebelli notch it overlaps the lingula from which it is separated by the precentral fissure laterally it extends along the upper and anterior part of each hemisphere where it forms a wing-like prolongation the ala lobula centralis the monticulus and quadrangula lobules the monticulus is the largest part of the superior vermus anteriorly it overlaps the lobulocentralis from which it is separated by the postcentral fissure laterally it is continuous with the quadrangula lobule in the hemispheres it is divided by the preclival fissure into an anterior raised part the cullmen and a posterior slope part the clivus the quadrangula lobula is similarly divided the cullmen and the anterior parts of the quadrangula lobules form the lobus cullmenus the clivus and the posterior parts the lobus clevi the pholum vermus and superior semi-lunar lobule the pholum vermus pholum cacuminus cacuminal lobe is a short narrow concealed band at the posterior extremity of the vermus consisting apparently of a single pholium but in reality marked on its upper and under surfaces by secondary fissures laterally it expands in either hemisphere into a considerable lobule the superior semi-lunar lobule lobule is semi-lunar superior posterior superior lobules which occupy the posterior third of the upper surface of the hemispheres and is bounded below by the horizontal sulcus the superior semi-lunar lobules and the pholium vermus form the lobus semilunaris the under surface of the cerebellum presents in the middle line the inferior vermus buried in the velecula and separated from the hemisphere on either side by a deep groove the sulcus veleculae here as on the upper surface there are more deep fissures dividing into separate segments of lobules but the arrangement is more complicated and the relation of the segments to the vermus to those of the hemispheres is less clearly marked the inferior vermus is subdivided from below backwards into one the nodule two the uvula three the pyramid and four the tumor vermus the corresponding parts on the hemisphere are the tonsila cerebelli three the biventrolobule and four the inferior semilunar lobule the three main fissures are one the post nodular fissure which runs transversely across the vermus between the nodule and the uvula in the hemisphere this fissure passes in front of the tonsil crosses between the flocculus in front and the biventrolobule behind and joins the anterior end of the horizontal sulcus two the pre-paramidal fissure crosses the vermus between the uvula in front and the pyramid behind then curves forward between the tonsil and the biventrolob to join the post nodular fissure three the post-paramidal fissure passes across the vermus between the pyramid and the tuber vermus and in the hemispheres courses behind the tonsil and biventrolobules and then along the lateral border of the biventrolobule the post-nodular sulcus in the hemisphere it forms the anterior boundary of the inferior semi-lunar lobule the nodule and flocculus the nodule nodulus vermus nodular lobe or anterior end of the inferior vermus a butts against the roof of the fourth ventricle and can only be distinctly seen after the cerebellum has been separated from the medulla oblongata and pons on either side of the nodule is a thin layer of white substance named the posterior medullary vellum it is semi-lunar informed its convex border being continuous with the white substance of the cerebellum it extends on either side as far as the flocculus the flocculus is a prominent irregular lobule situated in front of the biventrolobule between it and the midter cerebellar peduncle it is subdivided into a few small laminate and is connected to the inferior medullary vellum by it's central white core the flocculi together with the posterior medullary vellum and nodule constitute the lobis noduli the uvula and tonsilla the uvula uvula vermus uvula lobe forms a considerable portion of the inferior vermus it is separated on either side from the tonsil by the succulus folliculae at the bottom of which it is connected to the tonsil by a ridge of grey matter indented on it's surface by shallow furrows and hence called the furrowed band the tonsilla tonsila cerebelli amygdeline nucleus is a rounded mass situated in the hemispheres each lies in a deep fossa termed the bird's nest nidus aevis between the uvula and the biventrolobule the uvula and tonsillae form the lobis uvulae the pyramid and biventrolobules constitute the lobis parameters the pyramid is a conical portion forming the largest prominence of the inferior vermus it is separated from the hemispheres by the succus folliculae across which it is connected to the biventrolobule by an indistinct grey band analogous to the furrowed band already described the biventrolobule is triangular in shape it's apex points backwards and is joined by the grey band to the pyramid the lateral border is separated from the inferior semilunar lobule by the post-paramidal fissure the base is directed forward and is on a line with the anterior border of the tonsil and is separated from the flocculus by the post-nodular fissure the tuber vermus tuber valvulae and the inferior semilunar lobule lobulis semilunaris inferior posterior superior lobule collectively form the lobus tuberis tuberae lobe the tuber vermus the most posterior division of the interior vermus is of small size and laterally spreads out into the large inferior semilunar lobules which comprise at least two-thirds of the inferior surface of the hemisphere internal structure of the cerebellum the cerebellum consists of white and grey substance white substance if a sagittal section be made through either hemisphere the interior will be found to consist of a central stem of white substance in the middle of which is a grey mass the dentate nucleus from the surface of the central white stem a series of plates is prolonged these are covered with grey substance and form the laminate in consequence of the main branches the central stem dividing and subdividing a characteristic appearance named arbor vitae is presented if the sagittal section be made through the middle of the vermus it will be found that the central stem divides into a vertical and horizontal branch the vertical branch passes upwards to the colmen monticuli where it subdivides freely one of its ramifications passing forward and upward to the central lobule the horizontal branch passes backwards to the folium vermus greatly diminished in size in consequence of having giving off large secondary branches one from its upper surface ascends to the clivus monticuli the others descend and enter the lobes of the inferior vermus that is the tuber vermus the pyramid, the uvula and the nodule the white substance of the cerebellum include two sets of nerve fibers one projection fibers two fibrae propriae projection fibers the cerebellum is connected to other parts of the brain by three large bundles of projection fibers that is to the cerebrum by the superior peduncle to the pons by the middle peduncle and to the medulla oblongata by the inferior peduncles the superior cerebellar peduncles brachia conjuntiva two in number emerge from the upper and medial portions of the right substance of the hemispheres and are placed under cover of the upper part of the cerebellum they are joined to each other across the middle line by the anterior medullary vellum and can be followed upward as far as the inferior colliculi under which they disappear below they form the upper lateral boundaries of the fourth ventricle but as they ascend it converts on the dorsal aspect of the ventricle and thus assist in roofing it in the fibres of the superior peduncle are mainly derived from the cells of the dentate nucleus of the cerebellum and emerge from the high list of this nucleus a few arise from the cells of the smaller gray nuclei in the cerebellar white substance and others from the cells of the cerebellar cortex they are continued upward beneath the corpore quaggigemina and the fibers of the two peduncles undergo a complete decussation ventral to the sylvian aqueduct having crossed the middle line they divide into ascending and descending groups of fibers the former ending in the red nucleus the thalamus and the nucleus of the ocular motor nerve while the descending fibers can be traced as far as the dorsal part of the pons cahal believes them to be continued into the anterior finiculus of the medulla spinalis as already stated the majority of the fibers of the ventral spinal cerebellar vesiculus of the medulla spinalis pass to the cerebellum which they reach by way of the superior peduncle the middle cerebellar peduncles brachia pontis are composed entirely of centripetal fibers which arise from the cells of the nuclei pontis of the opposite side and end in the cerebellar cortex the fibers are arranged in three vesiculi superior inferior and deep the superior vesiculus the most superficial is derived from the upper transverse fibers of the pons it is directed backward and lateral where it is superficial to the other two vesiculi and is distributed mainly to the lobules on the inferior surface of the cerebellar hemisphere and to the parts of the superior surface adjoining the posterior and lateral margins the inferior vesiculus is formed by the lowest transverse fibers of the pons it passes under cover of the superior vesiculus and continues downward and backward more or less parallel with it to be distributed to the folia on the under surface close to the vermis the deep vesiculus comprises most of the deep transverse fibers of the pons it is at first covered by the superior and inferior vesiculi but crosses obliquely and appears on the medial side of the superior from which it receives a bundle its fibers spread out and pass to the upper anterior cerebellar folium the fibers of this vesiculus cover those of the restiform body the inferior cerebellar peduncles restiform bodies pass at first upward and lateral word forming part of the lateral walls of the fourth ventricle and then bend abruptly backwards to enter the cerebellum between the superior and middle peduncles each contains the following vesiculi one the dorsal spinocerebellar vesiculus of the medulla spinalis which ends mainly in the superior vermis two fibers from the gracillin cuneate nuclei of the same side and of the opposite sides three fibers from the opposite olivary nuclei four crossed and uncrossed fibers from the reticular formation of the medulla oblongata five vestibular fibers drive partly from the vestibular division of the acoustic nerve and partly from the nuclei in which this division ends these fibers occupy the medial segment of the inferior peduncle and divide into ascending and descending groups of fibers the ascending fibers partly end in the roof nucleus on the opposite side of the cerebellum six cerebello bulbar fibers which come from the opposite roof nucleus and probably from the dentate nucleus and are said to end in the nucleus of dors and in the formaceo reticularis of the medulla oblongata seven some fibers from the ventral spinocerebellar fasciculus are said to join the dorsal spinocerebellar fasciculus the anterior medullary vellum vellum medullare anterioris valve of the sends superior medullary vellum is a thin transparent lamina of white substance which stretches between the superior peduncle on the dorsal surface of its lower half the folia and lingula are prolonged it forms, together with the superior peduncle the roof of the upper part of the fourth ventricle it is narrow above where it passes beneath the inferior colliculi and broader below where it is continuous with the white substance of the superior vermis a slightly elevated ridge the frengillum velli descends upon its upper part from between the inferior colliculi and on either side of this the trochlear nerve emerges the posterior medullary vellum vellum medullare posterioris inferior medullary vellum is a thin layer of white substance prolonged from the white center of the cerebellum above and on either side of the nodule it forms a part of the roof of the fourth ventricle somewhat semi-lunar in shape its convex edge is continuous with the white substance of the cerebellum while its thin concave margin is apparently free in reality, however it is continuous with the epithelium of the ventricle which is prolonged downward from the posterior medullary vellum to the ligulae the two medullary vellum are in contact with each other along their line of emergence from the white substance of the cerebellum and this line of contact forms the summit of the roof of the fourth ventricle which in a vertical section through the cavity appears as a pointed angle the fibriae propriae of the cerebellum are of two kinds commissural fibres which cross the middle lines at the anterior and posterior parts of the vermis and connect the opposite halves of the cerebellum two arcuate or association fibres which connect adjacent laminate with each other gray substance the gray substance of the cerebellum is found in two situations one on the surface forming the cortex two as independent masses in the anterior one the gray substance of the cortex presents a characteristic foliated appearance due to the series of laminate which are given off from the central white substance these in turn give off secondary laminate which are covered by gray substance externally the cortex is covered by pia mater internally is the medullary center consisting mainly of nerve fibres end of section 10 section 11 of gray's anatomy part 4 this is a LibriVox recording all LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org recording by ML Cohen anatomy to human body part 4 by Henry Gray the hindbrain part 4 microscopic appearance of the cortex the cortex consists of two layers that is an external gray molecular layer and an internal rust colored nuclear layer between these is an incomplete stratum of cells which are characteristic of the cerebellum that is the cells of Purkinje the external gray or molecular layer consists of fibres and cells the nerve fibres are delicate fibrillae and are derived from the following sources a the dendrites and axon collaterals of Purkinje cells b fibres from the cells of the nuclear layer c fibres from the central white substance of the cerebellum d fibres derived from cells in the molecular layer itself in addition to these are other fibres which have a vertical direction and are the processes of the large neuroglial cells situated in the nuclear layer they pass outward to the periphery of the gray matter where they expand into little conical enlargements which form a sort of limiting membrane beneath the pia mater analogous to the membranolimitans interna in the retina formed by the sustentacular fibres of Mueller the cells of the molecular layer are small and are arranged in two striata an outer and an inner they all possess branched axons those of the inner layer are termed basket cells they run for some distance parallel with the surface of the folium given off collaterals which pass in a vertical direction towards the bodies of the Purkinje cells which they become enlarged and form basket-like networks the cells of Purkinje form a single stratum of large flasculate shells at the junction of the molecular and nuclear layers their bases resting against the latter in fishes and reptiles they are arranged in several layers the cells are flattened in a direction transverse to the long axis of the folium and thus appear broad in sections carried across the folium and fuse a form in sections parallel to the long axis of the folium from the neck of the flask one or more dendrites arise and pass into the molecular layer where they subdivide and form an extremely rich arborescence the various subdivisions of the dendrites being covered by lateral spine-like processes the arborescence is not circular but like the cell is flattened at right angles to the long axis of the folium in other words it does not resemble a round bush a fruit tree trained against a trellis or a wall hence in sections carried across the folium the arborescence is broad and expanded where in those which are parallel to the long axis of the folium the arborescence like the cell itself is seen in profile and is limited to a narrow area from the bottom of the flask-shaped cell the axon arises this passes through the nuclear layer and becoming medulated is continued as a nerve fiber as this axon traverses the granular layer it gives off fine collaterals some of which run back into the molecular layer the internal rust-colored or nuclear layer is characterized by containing numerous small nerve cells of a reddish-brown color together with many nerve fibrils most of the cells are nearly spherical and provided with short dendrites which spread out in a spider-like manner in the nuclear layer their axons pass outward into the molecular layer and bifurcating at right angles run for some distance parallel with the surface in the outer part of the nuclear layer are some larger cells of the type 2 of Golgi their axons undergo frequent division as soon as they leave the nerve cells and pass into the nuclear layer while the dendrites ramified chiefly in the molecular layer finally, in the gray substance of the cerebellar cortex there are fibers which come from the white center and penetrate the cortex the cell origin of these fibers is unknown though it is believed that it is probably in the gray substance of the medulla spinalis some of these fibers end in the nuclear layer by dividing into numerous branches on which are to be seen peculiar moss-like appendages hence, they have been turned by Ramon Icahal the moss fibers they form an arborescence around the cells of the nuclear layer and are said to come from fibers other fibers, the clinging or tendril fibers derived from the medullary center can be traced into the molecular layer where their branches cling around the dendrites of Purkinje cells they are said to come from fibers of the middle peduncle 2. the independent centers of gray substance in the cerebellum are four in number on either side one is of large size and is known as the nucleus dentatus the other three, much smaller are situated near the middle of the cerebellum and are known as the nucleus emboliformis nucleus globosis and nucleus vestigii the nucleus dentatus is situated a little to the medial side to the center of the stem of the white substance of the hemisphere it consists of an irregularly folded lamina of grayish-yellow color containing white fibers and presenting on its antiremedal aspect an opening, the hilus from which most of the fibers of the superior pinnacle emerge the nucleus emboliformis lies immediately to the medial side of the nucleus dentatus and partly covering its hilus the nucleus globosis is an elongated mass directed anterior posteriorly and placed medial to the proceeding the nucleus vestigii is somewhat larger than the other two and is situated close to the middle line at the anterior end of the superior vermus and immediately over the roof of the fourth ventricle a thin layer of white substance the cerebellum is concerned with the coordination of movements necessary in equilibration, locomotion and prehension in it terminate pathways conducting impulses of muscle sense, tendon sense joint sense and equilibatory disturbances with the exception of the ventral spinocerebellar fasciculus these impulses enter through the inferior pinnacle the reflex arc is completed by fibers in the superior pinnacle which pass to the red nucleus and thence by additional neurons rubro spinal tract to the motor centers the exact function of its different parts is still quite uncertain owing to the contradictory nature of the evidence furnished by 1. ablation experiments upon animals and 2. clinical observation in man of the effects produced by abscesses or tumors affecting different portions of the organ the fourth ventricle ventriculus cortis the fourth ventricle or cavity of the hindbrain is situated in front of the cerebellum and behind the pons and upper half of the medulla oblongata developmentally considered the fourth ventricle consists of three parts a superior belonging to the isthmus rhombincephali an intermediate to the metencephalon and an inferior to the myelincephalon it is lined by ciliated epithelium and is continuous below with the central canal of the medulla oblongata above it communicates by means of a passage termed a cerebral aqueduct with the cavity of the third ventricle it presents four angles and possesses a roof or dorsal wall a floor or a ventral wall and lateral boundary angles the superior angle is on a level with the upper border of the pons and is continuous with the lower end of the cerebral aqueduct the inferior angle is on a level with the lower end of the olive and opens into the central canal of the medulla oblongata each lateral angle corresponds with the point of meeting of the brachia and inferior peduncle a little below the lateral angles on a level with the strea medullaries the ventricular cavity is prolonged outward in the form of two narrow lateral recesses one on either side these are situated between the inferior peduncles and the flocculi and reaches far as the attachments of the glossopharyngeal and vegas nerves lateral boundaries the lower part of each lateral boundary is constituted by the clava the fasciculus cuneatus and the inferior peduncle the upper part by the middle and the superior peduncle roof or dorsal wall the upper portion of the roof is formed by the superior peduncle and the anterior medullary vellum the lower portion by the posterior medullary vellum the epithelial lining of the ventricle covered by the telecoroidia inferior the tainé of the fourth ventricle and the obex the superior peduncle emerging from the central white substance of the cerebellum pass upward and forward forming at first the lateral boundaries of the upper part of the cavity on approaching the inferior colliculi they converge and their medial portions overlap the cavity and form part of its roof the anterior medullary vellum fills the angular interval between the superior peduncle and is continuous behind with the central white substance of the cerebellum it is covered on its dorsal surface by the lingula of the superior vermis the posterior medullary vellum is continued downward and forward from the central white substance of the cerebellum in front of the nodule and tonsils and ends inferiorly in a thin concave somewhat ragged margin below this margin the roof is devoid of nervous matter except in the immediate vicinity of the lower lateral boundaries of the ventricle where two narrow white bands the tainé of the fourth ventricle ligulae appear these bands meet over the inferior angle of the ventricle in a thin triangular lamina the obex the non-nervous part of the roof is formed by the epithelial lining of the ventricle which is prolonged downward as a thin membrane from the deep surface of the posterior medullary vellum to the corresponding surface of the obex and tainé and thence on to the floor of the ventricular cavity it is covered and strengthened by a portion of the pia mater which is named the tila cordia of the fourth ventricle the tainé of the fourth ventricle tainé ventricleicorto ligulae are two narrow bands of white matter one on either side which complete the lower part of the roof of the cavity each consists of a vertical and a horizontal part the vertical part is continuous below the obex with the clava to which it is inherent by lateral border the horizontal portion extends transversely across the inferior peduncle below the strié medullaris and roofs in the lower and posterior part of the lateral recess it is attached by its lower margin to the inferior peduncle and partly encloses the cori plexus which however projects beyond it like a cluster of grapes and hence this part of the tainé has been termed the corny copia the obex is a thin triangular gray lamina which roofs in the lower angle of the ventricle and is attached by its lateral margins to the clavae the tila cordia of the fourth ventricle the pia mater which is carried upward between the cerebellum and the medulla oblongata it consists of two layers which are continuous with each other in front and are more and less adherent throughout the posterior layer covers the anterior inferior surface of the cerebellum while the anterior is applied to the structures which form the lower part of the roof of the ventricle and is continued inferiorly with the pia mater on the inferior peduncles and closed part of the medulla cori plexus these consists of two highly vascular inflections of the tila cordia which invaginate the lower part of the roof of the ventricle and are everywhere covered by the epithelial lining of the cavity each consists of a vertical and a horizontal portion the former lies close to the middle line and the latter passes into the lateral recess and projects beyond its apex the vertical parts of the plexus are distinct from each other but the horizontal portions are joined in the middle line and hence the entire structure presents the form of the letter T the vertical limb of which however is double openings in the roof in the roof of the fourth ventricle there are three openings a medial and two lateral the medial aperture is situated immediately above the inferior angle of the ventricle the lateral apertures are found at the extremities of the lateral recesses by means of these three openings the ventricle communicates with the subarachnoid cavity and the cerebral spinal fluid can circulate from the one to the other rhomboid fossa fossa rhomboidia floor of the fourth ventricle the anterior part of the fourth ventricle is named from its shape the rhomboid fossa and its anterior wall formed by the back of the pons and medulla oblongata constitutes the floor of the ventricle it is covered by a thin layer of gray substance continuous with that of the medulla spinalis superficial to this is a thin lamina of neuroglia which constitutes the appendema of the ventricle and supports a layer of ciliated epithelium the fossa consists of three parts superior, intermediate and inferior the superior part is triangular in shape and limited laterally by the superior cerebellum peduncle the inferior part is triangular and its downwardly directed apex directed upward is continuous with the cerebral aqueduct its base is repedented by an imaginary line at the level of the upper ends of the superior fovea the intermediate part extends from this level to that of the horizontal portions of the tinea of the ventricle it is narrow above where it is limited laterally by the medulla peduncle but widens below and is prolonged into the lateral recesses of the ventricle the inferior part is triangular the directed apex named the calamus scriptorius is continuous with the central canal of the closed port of the medulla oblongata the rhomboid fossa is divided into symmetrical halves by a median sulcus which reaches from the upper to the lower angles of the fossa and is deeper below than above on either side of the sulcus is an elevation the medial eminence bounded laterally by a sulcus the sulcus limitans the inferior part of the fossa the medial eminence has a width equal to that of the corresponding half of the fossa but opposite the superior fovea it forms an elongated swelling the colliculus facialis which overlies the nucleus of the abducent nerve and is impart at least produced by the ascending portion of the root of the facial nerve in the inferior part of the fossa the medial eminence assumes the form of a triangular area the trigonum hypoglossae when examined under water with a lens this trigon is seen to consist of a medial and a lateral area separated by a series of oblique furrows the medial area corresponds with the upper part of the nucleus of the hypoglossal nerve the lateral with a small nucleus the nucleus intercalatus the sulcus limitans forms the lateral boundary of the medial eminence in the superior part of the rhomboid fossa it corresponds with the lateral limit of the fossa and presents in this gray area the locus ceruleus which owes its color to an underlying patch of deeply pigmented nerve cells termed the substancia furuginia at the level of the colliculus facialis the sulcus limitans widens into a flattened depression the superior fovea and in the inferior part of the fossa appears as a distinct dimple the inferior fovea lateral to the fovea is around the elevation named the area acustica widens into the lateral recess and there forms a feebly marked swelling the tuberculum acusticum winding around the inferior peduncle and crossing the area acustica and the medial eminence are a number of white strands the stream edularis which form a portion of the cochlear division of the acoustic nerve and disappear into the median sulcus below the inferior fovea and between the trigonum hypoglossi and the lower part of the area acustica in the inferior dark field the alicinuria which corresponds to the sensory nucleus of the vagus and clausopharyngeal nerves the lower end of the alicinuria is crossed by a narrow translucent ridge the finiculus separans and between this finiculus and the clava is a small tongue shaped area the area post trima on section it is seen that the finiculus separans is formed by a strip of thickened appendema and the area post trima by loose highly vascular neuroglial tissue containing nerve cells of moderate size end of section 11 section 12 of gray's anatomy part 4 this is a LibriVox recording all LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org recording by ML Cohen anatomy of the human body part 4 by Henry Gray the midbrain or mesencephalon the midbrain or mesencephalon is the short constricted portion which connects the pons and cerebellum with the thalamencephalon and cerebral hemispheres it is directed upward and forward and consists of 1. a ventrolateral portion composed of a pair of cylindrical bodies named as cerebral peduncles 2. a dorsal portion consisting of 4 rounded eminences named the corpora quadragemina and 3. an intervening passage or tunnel the cerebral aqueduct which represents the original cavity of the midbrain and connects to 3rd with the 4th ventricles the cerebral peduncles pedunculus cerebrae cruise cerebrae are two cylindrical masses situated at the base of the brain and largely hidden by the temporal lobes of the cerebrum which must be drawn aside or removed in order to expose them they emerge from the upper surface of the pons one on either side of the middle line and diverging as they pass upward and forward disappear into the substance of the cerebral hemispheres the depressed area between the crura is termed the interpeduncular fossa and consists of a layer of grayish substance the posterior perforated substance which is pierced by small apertures for the transmission of blood vessels its lower part lies on the ventral aspect of the medial portions of the tegmenta and contains a nucleus named the interpeduncular ganglion its upper part assists in forming the floor of the 3rd ventricle the ventral surface of each peduncle is crossed from the medial to the lateral side by the superior cerebellar and posterior cerebral arteries its lateral surface is in relation to the gyrus hippocampi of the cerebral hemisphere and is crossed from behind forward by the trochlear nerve to the point of disappearance of the peduncle into the cerebral hemisphere the optic tract winds forward around its ventral lateral surface the medial surface of the peduncle forms a lateral boundary of the interpeduncular fossa and is marked by a longitudinal furrow the ocular motor sulcus from which the roots of the ocular motor nerve emerge on the lateral surface of each peduncle there is a second longitudinal furrow termed a lateral sulcus the miscus come to the surface in this sulcus and pass backward and upward to disappear under the inferior caliculus structure of the cerebral peduncles on transverse section each peduncle is seen to consist of a dorsal and the ventral part separated by a deeply pigmented lamina of gray substance turn the substantia nigra the dorsal part is named the tegmentum the ventral, the base or crustum the two bases are separated from each other but the tegmenta are joined in the medium plane by a forward prolongation of the raffae of the pons laterally, the tegmenta are free, dorsally they blend with the corpore quaggigemina the base base's pedunculae, crusta or pez, is semi-lunar on transverse section and consists almost entirely of longitudinal bundles of effrin fibers which arise from the cells of the cerebral cortex are grouped into three principal sets that is cerebrospinal, frontopontine and temporopontine the cerebrospinal fibers derive from the cells of the motor area of the cerebral cortex occupy the middle three fifths of the base they are continued partly to the nuclei of the motor cranial nerves but mainly into the pyramids of the medulla oblongata the frontopontine fibers are situated in the medial fifth of the base they arise from the cells of the frontal lobe and end in the nuclei of the pons the temporopontine fibers are lateral to the cerebrospinal fibers they originate in the temporal lobe and end in the nuclei pontus the substantia nigra intercalatum is a layer of gray substance containing numerous deeply pigmented multipolar nerve cells it is semi-lunar on transverse section, it's concavity being directed towards tegmentum from its convexity, prolongations extend between the fibers of the base of the peduncle thicker medially than laterally it reaches from the oculomotor sulcus to the lateral sulcus and extends from the upper surface of the pons to the subthalamic region its medial part is traversed by the fibers of the oculomotor nerve as these stream forward to reach the oculomotor sulcus the connections of the cells of the substantia nigra have not been definitively established but it receives collaterals from the medial amniscus and the pyramidal bundles bectoro is of the opinion that the fibers from the motor area of the cerebral cortex forps synapses with the cells whose axons pass to the motor nucleus of the trigeminal nerve and serve for the coordination of the muscles of mastication the tegmentum is continuous below with the reticular formation of the pons and like it consists of longitudinal and transverse fibers together with a considerable amount of gray substance the principal gray masses of the tegmentum are the red nucleus and the introduncular ganglion of its fibers the chief longitudinal tracts are the superior peduncle the medial longitudinal fasciculus and the lamniscus gray substance the red nucleus is situated in the anterior part of the tegmentum and is continued upward into the posterior part of the subthalamic regions in sections at the level of the superior it appears as a circular mass which is traversed by the fibers of the ocular motor nerve it receives many terminals and collaterals from the superior cerebellum peduncle also collaterals from the ventral longitudinal bundle from goudin's bundle and the median lamniscus the axons of its larger cells cross the middle line and are continued downward into the lateral finiculus of the medulla spinalis as the rubro spinal tract those of its smaller cells and mainly in the thalamus the rubro spinal tract forms an important part of the pathway from the cerebellum to the lower motor centers the interpeduncular ganglion is a median collection of nerve cells situated in the ventral part of the tegmentum the fibers of the fasciculus retroflexus of minord which have their origin in the cells of the ganglion hebenula end in it besides the two nuclei mentioned there are small collections of cells which form the dorsal and ventral nuclei and the central nucleus or nucleus of the raffae white substance one, the origin and course of the superior peduncle have already been described two, the medial posterior longitudinal fasciculus is continuous below with the proper fasciculi of the anterior and lateral finiculi of the medulla spinalis in the medulla oblongata it runs close to the middle line near the floor of the fourth ventricle in the midbrain it is situated on the ventral aspect of the cerebral aqueduct below the nuclei of the ocular motor and trochlear nerves its connections are imperfectly known but it consists largely of ascending and descending intersegmental or association fibers which connect in the nuclei of the hindbrain and midbrain to each other many of the fibers arise in Dieter's nucleus that are vestibular nucleus and divide into ascending and descending branches which send terminals and collaterals to the motor nuclei of the cranial and spinal nerves its spinal portion is located in the anterior finiculus and is known as the vestibular spinal fasciculus other fibers pass to the median longitudinal bundle from cells in the reticular formation of the medulla, pons and midbrain and also from certain large cells in the terminal nucleus of the trigeminal nerve according to Edinger, it extends to the so-called nucleus of the posterior longitudinal bundle in the hypothalamic region but this is uncertain and the fibers above the nucleus of the ocular motor are smaller in diameter than the rest of the bundle according to Held fibers from the posterior commissure can be traced into the posterior longitudinal bundle and according to the same author many of the descending fibers arise in the superior colliculus and after decasating in the middle line and in the motor nuclei of the pons and medulla oblongata these fibers from the superior colliculus probably pass into the ventral longitudinal bundle fibers are said to pass through the medial longitudinal fasciculus from the nucleus of the abducent into the ocular motor nerve of the opposite side and through this nerve to the rectus medialis oculi phrasier, however, denies the existence of such fibers again, fibers are said to be prolonged through this fasciculus from the nucleus of the ocular motor nerve into the facial nerve and are distributed to the albricularis oculi the torogator and the frontalis the ventral longitudinal bundle consists for the most part of the tectospinal fasciculus and arises from the superior colliculus the fibers arch ventrally around the central gray matter and cross the midline in the fountain-decasation of Manert and then descend in the tegmentum part of them passing through the red nucleus ventral to the medial longitudinal bundle in the medulla oblongata and spinal cord its fibers are more or less intermingled with the medial longitudinal bundle and the rubro spinal tract it descends in the adjoining region of the ventral and lateral finiculi collaterals and terminals are given off to the red nucleus and probably other nuclei of the brainstem and to the anterior column of the spinal cord it is probably concerned in optic reflexes the medial lemniscus or medial filet the fibers of the medial lemniscus take origin in the gracil and cuneate nuclei of the medulla oblongata and as internal arcuate fibers they cross to the opposite side in the sensory decacation they then pass into inter-olivary stratum upward through the medulla oblongata in which they are situated behind the cerebrospinal fibers and between the olives in the pons and lower part of the midbrain it occupies the ventral part of the reticular formation and tegmentum close to the raffae while above it gradually shifts to the dorsolateral part of the tegmentum in the angle between the red nucleus and the substantia nigra in the pons it assumes a flattened ribbon-like appearance and is placed dorsal to the trapezium as the lemniscus ascends it receives additional fibers from the terminal sensory nuclei to the opposite side many of the fibers which arise from the terminal sensory nuclei of the cranial nerves pass upward in the formaceo-articularis as a separate bundle known as the central tract of the cranial nerves to the thalamus many fibers either terminate in or send off collaterals to the gray matter of the medulla, the pons and the midbrain large numbers of fibers pass to or from the substantia nigra many collaterals enter the red nucleus and other fibers are said to run in the ocular nucleus the great bulk of the fibers however enter the ventral lateral portion of the thalamus give off collaterals to the posterior semi-lumer nucleus and then terminate in the principal central nucleus of the thalamus in the cerebral peduncle a few of its fibers pass upward in the lateral part of the base of the peduncle on the dorsal aspect of the temporal pontine fibers and reach the lentiform nucleus and the insula the greater part of the medialamniscus on the other hand is prolonged through the tegmentum and most of its fibers end in the thalamus probably some are continued directly through the occipital part of the internal capsule through the cerebral cortex from the cells of the thalamus a relay of fibers is prolonged to the cerebral cortex the medialamniscus may be considered as the upward continuation of the posterior finiculus of the spinal cord and to convey conscious impulses of muscle sense the central orthalamic tract of the cranial nerves is closely associated with the medialamniscus the fibers of the spinal thalamic fasciculi are continued from the spinal cord into this tract which passes upward in the reticular formation and the tegmentum to the thalamus along the dorsal side of the medialamniscus it receives fibers from the opposite terminal sensor nuclei the vagus, glossopharyngeal facial, trigeminal and probably the vestibular nerves many of the secondary sensory fibers of the trigeminal cross the raffae from its terminal nucleus and pass upward to the thalamus by a more or less separate but closely associated pathway known as the central tract of the trigeminal nerve which also lies in the dorsal aspect of the lamniscus these two tracts give off collaterals to the posterior semilunar nucleus of the thalamus and terminate in the anterior semilunar nucleus of the ventrolateral region of the thalamus sending collaterals to the trisona inserta the fibers of the rubra spinal tract bundle of monocow arise in the red nucleus cross the midline in a decassation of ferrel and pass downward in the formatior reticularis of the brainstem to the lateral finiculus of the spinal cord ventral to the cross pyramidal tract the lateral lamniscus lamniscus lateralis comes to the surface of the midbrain along its lateral sulcus and disappears under the inferior colliculus it consists of fibers from the terminal nuclei of the cochlear division of the acoustic nerve together with others from the superior olivary and trapezoid nuclei most of these fibers are crossed but some are uncrossed many of them pass to the inferior colliculus of the same or opposite side but others are prolonged to the thalamus and then through the occipital part of the internal capsule to the middle and superior temporal gyri the corporeal quadragemina are four rounded eminences which form the dorsal part of the midbrain they are situated above and in front of the anterior medullary vellum and superior peduncle and below and behind the third ventricle and posterior commissure they are covered by the splenium of the corpus callosum and are partly overlapped on either side by the medial angle or pulvanar of the posterior end of the thalamus on the lateral aspect, under cover of the pulvanar is an oval eminence named the corporeal quadragemina are arranged in pairs superior and inferior colliculi and are separated from one another by a crucial sinus the longitudinal part of the sulcus expands superiorly to form a slight depression which supports the pineal body a cone-like structure which project backwards from the thalamus cephalon and partly obscures the superior colliculi from the inferior end of the longitudinal sulcus a white band termed ephrenulum vely is prolonged downward to the anterior medullary vellum on either side of this band the trochlear nerve emerges and passes forward on the lateral aspect of the cerebral peduncle to reach the base of the brain the superior colliculi are larger and darker in color than the inferior and are oval in shape the inferior colliculi are hemispherical and somewhat more prominent than the superior the superior colliculi are associated with the sense of sight and the inferior with that of hearing from the lateral aspect of each colliculus a white band termed a brachium is prolonged upward and forward the superior brachium extends lateral word from the superior colliculus and passing between the pulvanar and the medial geniculate body is partly continued into an eminence called the lateral geniculate body and partly into the optic tract the inferior brachium passes forward and upward from the inferior colliculus disappears under cover of the medial geniculate body in close relationship with the corpora quadragemina are the superior peduncles which emerge from the upper and medial parts of the cerebellar hemispheres they run upward and forward and passing under the inferior colliculi enter the tegmenta as already described structure of the corpora quadragemina the inferior colliculus inferior inferior quadrageminal body post-gemina consists of a compact nucleus of gray substance containing large and small multipolar nerve cells and more or less completely surrounded by white fibers derived from the lateral lemniscus most of these fibers end in the gray nucleus of the same side but some cross the middle line and end in that of the opposite side from the cells of the gray nucleus fibers are prolonged through the inferior brachium into the tegmentum of the cerebral peduncle and are carried to the thalamus and the cortex of the temporal lobe and other fibers cross the middle line and end in the opposite colliculus the superior colliculus colliculus superior superior quadrageminal body pre-gemina is covered by a thin stratum stratum zonale of white fibers the majority of which are derived from the optic tract beneath this is the stratum synerium a cap-like layer of gray substance thicker in the center than at the circumference and consisting of numerous small multipolar nerve cells embedded in a fine network of nerve fibers still deeper is the stratum opticum containing large multipolar nerve cells separated by numerous fine nerve fibers finally there is the stratum lemnisci consisting of fibers derived partly from the lemniscus and partly from the cells of the stratum opticum interspersed among these fibers are many large multipolar nerve cells the two last name strata are sometimes termed the gray-white layers from the fact they consist of both gray and white substance of the afferent fibers which reach the superior colliculus some are derived from the lemniscus but the majority have their origins in the retina and are conveyed to it through the superior brachium all of them end by arborizing around the cells of the gray substance of the efferent fibers some cross the middle line to the opposite colliculus many ascend through the superior brachium and finally reach the cortex of the occipital lobe while others, after undergoing dequisition fountain dequisition of minert form the tectospinal fasciculus which descends through the formacea reticularis of the midbrain, pons, and medulla oblongata into the medulla spinalis where it is found partly in the anterior finiculus and partly intermingled with the fibers of the rubrospinal tract the corpora quadragemina are larger in the lower animals than in man in fishes, reptiles, and birds they are hollow and only to a number corpora bigemina they represent the superior colliculi of mammals and are frequently termed the optic lobes because of their intimate connection with the optic tracts the cerebral aqueduct aqueductus cerebrae, aqueductus sylveus is a narrow canal about 15 millimeters long situated between the corpora quadragemina and tegmenta and connecting the third with the fourth ventricle its shape, as seen in transverse section varies at different levels being T-shaped, triangular above and oval in the middle the central part is slightly dilated and was named by Retsius the ventricle of the midbrain it is lined by ciliated columnar epithelium and is surrounded by a layer of gray substance named the central gray stratum this is continuous below with the gray substance in the rhombord fossa and above with that of the third ventricle dorsally it is partly separated from the gray substance of the quadrageminal bodies by the fibers of the lemniscus ventral tuid are the medial longitudinal fasciculus and the formacea reticularis of the tegmentum scattered throughout the central gray stratum are numerous nerve cells of various sizes interlaced by a network of fine fibers besides these scattered cells it contains three groups which constitute the nuclei of the oculomotor and trochlear nerves and the nucleus of the mesencephalic root of the trigeminal nerve the nucleus of the trigeminal nerve extends along the entire length of the aqueduct and occupies the lateral part of the gray stratum while the nuclei of the oculomotor and trochlear nerves are situated in its ventral part the nucleus of the oculomotor nerve is about 10 millimeters long and lies under the superior colliculus beyond which however it extends for a short distance into the gray substance of the third ventricle the nucleus of the trochlear nerve is small and nearly circular and is on a level with the plane carried transversely to the upper part of the inferior colliculus End of section 12