Anatomy_Gray

Anatomy_Gray. Inferiorly, the costal pleura reflects onto the diaphragm above the costal margin. In the midclavicular line, the pleural cavity extends inferiorly to approximately rib VIII. In the midaxillary line, it extends to rib X. From this point, the inferior margin courses somewhat horizontally, crossing ribs XI and XII to reach vertebra TXII. From the midclavicular line to the vertebral column, the inferior boundary of the pleura can be approximated by a line that runs between rib VIII, rib X, and vertebra TXII. The visceral pleura is continuous with the parietal pleura at the hilum of each lung, where structures enter and leave the organ. The visceral pleura is firmly attached to the surface of the lung, including both opposed surfaces of the fissures that divide the lungs into lobes. Although the visceral pleura is innervated by visceral afferent nerves that accompany bronchial vessels, pain is generally not elicited from this tissue.


Anatomy_Gray

Anatomy_Gray. Although the visceral pleura is innervated by visceral afferent nerves that accompany bronchial vessels, pain is generally not elicited from this tissue. The lungs do not completely fill the anterior or posterior inferior regions of the pleural cavities (Fig. 3.40). This results in recesses in which two layers of parietal pleura become opposed. Expansion of the lungs into these spaces usually occurs only during forced inspiration; the recesses also provide potential spaces in which fluids can collect and from which fluids can be aspirated. Anteriorly, a costomediastinal recess occurs on each side where costal pleura is opposed to mediastinal pleura. The largest is on the left side in the region overlying the heart (Fig. 3.40).


Anatomy_Gray

Anatomy_Gray. Anteriorly, a costomediastinal recess occurs on each side where costal pleura is opposed to mediastinal pleura. The largest is on the left side in the region overlying the heart (Fig. 3.40). The largest and clinically most important recesses are the costodiaphragmatic recesses, which occur in each pleural cavity between the costal pleura and diaphragmatic pleura (Fig. 3.40). The costodiaphragmatic recesses are the regions between the inferior margin of the lungs and inferior margin of the pleural cavities. They are deepest after forced expiration and shallowest after forced inspiration.


Anatomy_Gray

Anatomy_Gray. During quiet respiration, the inferior margin of the lung crosses rib VI in the midclavicular line and rib VIII in the midaxillary line, and then courses somewhat horizontally to reach the vertebral column at vertebral level TX. Thus, from the midclavicular line and around the thoracic wall to the vertebral column, the inferior margin of the lung can be approximated by a line running between rib VI, rib VIII, and vertebra TX. The inferior margin of the pleural cavity at the same points is rib VIII, rib X, and vertebra TXII. The costodiaphragmatic recess is the region between the two margins. During expiration, the inferior margin of the lung rises and the costodiaphragmatic recess becomes larger. The two lungs are organs of respiration and lie on either side of the mediastinum surrounded by the right and left pleural cavities. Air enters and leaves the lungs via main bronchi, which are branches of the trachea.


Anatomy_Gray

Anatomy_Gray. The pulmonary arteries deliver deoxygenated blood to the lungs from the right ventricle of the heart. Oxygenated blood returns to the left atrium via the pulmonary veins. The right lung is normally a little larger than the left lung because the middle mediastinum, containing the heart, bulges more to the left than to the right. Each lung has a half-cone shape, with a base, apex, two surfaces, and three borders (Fig. 3.43). The base sits on the diaphragm. The apex projects above rib I and into the root of the neck. The two surfaces—the costal surface lies immediately adjacent to the ribs and intercostal spaces of the thoracic wall. The mediastinal surface lies against the mediastinum anteriorly and the vertebral column posteriorly and contains the comma-shaped hilum of the lung, through which structures enter and leave.


Anatomy_Gray

Anatomy_Gray. The three borders—the inferior border of the lung is sharp and separates the base from the costal surface. The anterior and posterior borders separate the costal surface from the medial surface. Unlike the anterior and inferior borders, which are sharp, the posterior border is smooth and rounded. The lungs lie directly adjacent to, and are indented by, structures contained in the overlying area. The heart and major vessels form bulges in the mediastinum that indent the medial surfaces of the lung; the ribs indent the costal surfaces. Pathology, such as tumors, or abnormalities in one structure can affect the related structure.


Anatomy_Gray

Anatomy_Gray. The root of each lung is a short tubular collection of structures that together attach the lung to structures in the mediastinum (Fig. 3.44). It is covered by a sleeve of mediastinal pleura that reflects onto the surface of the lung as visceral pleura. The region outlined by this pleural reflection on the medial surface of the lung is the hilum, where structures enter and leave. A thin blade-like fold of pleura projects inferiorly from the root of the lung and extends from the hilum to the mediastinum. This structure is the pulmonary ligament. It may stabilize the position of the inferior lobe and may also accommodate the down-and-up translocation of structures in the root during breathing. In the mediastinum, the vagus nerves pass immediately posterior to the roots of the lungs, while the phrenic nerves pass immediately anterior to them.


Anatomy_Gray

Anatomy_Gray. In the mediastinum, the vagus nerves pass immediately posterior to the roots of the lungs, while the phrenic nerves pass immediately anterior to them. Within each root and located in the hilum are: a pulmonary artery, two pulmonary veins, a main bronchus, bronchial vessels, nerves, and lymphatics. Generally, the pulmonary artery is superior at the hilum, the pulmonary veins are inferior, and the bronchi are somewhat posterior in position. On the right side, the lobar bronchus to the superior lobe branches from the main bronchus in the root, unlike on the left where it branches within the lung itself, and is superior to the pulmonary artery. The right lung has three lobes and two fissures (Fig. 3.45A). Normally, the lobes are freely movable against each other because they are separated, almost to the hilum, by invaginations of visceral pleura. These invaginations form the fissures:


Anatomy_Gray

Anatomy_Gray. The oblique fissure separates the inferior lobe (lower lobe) from the superior lobe and the middle lobe of the right lung. The horizontal fissure separates the superior lobe (upper lobe) from the middle lobe. The approximate position of the oblique fissure on a patient, in quiet respiration, can be marked by a curved line on the thoracic wall that begins roughly at the spinous process of the vertebra TIV level of the spine, crosses the fifth interspace laterally, and then follows the contour of rib VI anteriorly (see pp. 241–242). The horizontal fissure follows the fourth intercostal space from the sternum until it meets the oblique fissure as it crosses rib V. The orientations of the oblique and horizontal fissures determine where clinicians should listen for lung sounds from each lobe.


Anatomy_Gray

Anatomy_Gray. The orientations of the oblique and horizontal fissures determine where clinicians should listen for lung sounds from each lobe. The largest surface of the superior lobe is in contact with the upper part of the anterolateral wall and the apex of this lobe projects into the root of the neck. The surface of the middle lobe lies mainly adjacent to the lower anterior and lateral wall. The costal surface of the inferior lobe is in contact with the posterior and inferior walls. When listening to lung sounds from each of the lobes, it is important to position the stethoscope on those areas of the thoracic wall related to the underlying positions of the lobes (see p. 243). The medial surface of the right lung lies adjacent to a number of important structures in the mediastinum and the root of the neck (Fig. 3.45B). These include the: heart, inferior vena cava, superior vena cava, azygos vein, and esophagus.


Anatomy_Gray

Anatomy_Gray. The right subclavian artery and vein arch over and are related to the superior lobe of the right lung as they pass over the dome of the cervical pleura and into the axilla. The left lung is smaller than the right lung and has two lobes separated by an oblique fissure (Fig. 3.46A). The oblique fissure of the left lung is slightly more oblique than the corresponding fissure of the right lung. During quiet respiration, the approximate position of the left oblique fissure can be marked by a curved line on the thoracic wall that begins between the spinous processes of vertebrae TIII and TIV, crosses the fifth interspace laterally, and follows the contour of rib VI anteriorly (see pp. 241–242). As with the right lung, the orientation of the oblique fissure determines where to listen for lung sounds from each lobe.


Anatomy_Gray

Anatomy_Gray. As with the right lung, the orientation of the oblique fissure determines where to listen for lung sounds from each lobe. The largest surface of the superior lobe is in contact with the upper part of the anterolateral wall, and the apex of this lobe projects into the root of the neck. The costal surface of the inferior lobe is in contact with the posterior and inferior walls. When listening to lung sounds from each of the lobes, the stethoscope should be placed on those areas of the thoracic wall related to the underlying positions of the lobes (see p. 243). The inferior portion of the medial surface of the left lung, unlike the right lung, is notched because of the heart’s projection into the left pleural cavity from the middle mediastinum. From the anterior border of the lower part of the superior lobe a tongue-like extension (the lingula of the left lung) projects over the heart bulge.


Anatomy_Gray

Anatomy_Gray. From the anterior border of the lower part of the superior lobe a tongue-like extension (the lingula of the left lung) projects over the heart bulge. The medial surface of the left lung lies adjacent to a number of important structures in the mediastinum and root of the neck (Fig. 3.46B). These include the: heart, aortic arch, thoracic aorta, and esophagus. The left subclavian artery and vein arch over and are related to the superior lobe of the left lung as they pass over the dome of the cervical pleura and into the axilla.


Anatomy_Gray

Anatomy_Gray. The left subclavian artery and vein arch over and are related to the superior lobe of the left lung as they pass over the dome of the cervical pleura and into the axilla. The trachea is a flexible tube that extends from vertebral level CVI in the lower neck to vertebral level TIV/V in the mediastinum where it bifurcates into a right and a left main bronchus (Fig. 3.47). The trachea is held open by C-shaped transverse cartilage rings embedded in its wall—the open part of the C facing posteriorly. The lowest tracheal ring has a hook-shaped structure, the carina, that projects backward in the midline between the origins of the two main bronchi. The posterior wall of the trachea is composed mainly of smooth muscle.


Anatomy_Gray

Anatomy_Gray. Each main bronchus enters the root of a lung and passes through the hilum into the lung itself. The right main bronchus is wider and takes a more vertical course through the root and hilum than the left main bronchus (Fig. 3.47A). Therefore, inhaled foreign bodies tend to lodge more frequently on the right side than on the left. The main bronchus divides within the lung into lobar bronchi (secondary bronchi), each of which supplies a lobe. On the right side, the lobar bronchus to the superior lobe originates within the root of the lung. The lobar bronchi further divide into segmental bronchi (tertiary bronchi), which supply bronchopulmonary segments (Fig. 3.47B).


Anatomy_Gray

Anatomy_Gray. The lobar bronchi further divide into segmental bronchi (tertiary bronchi), which supply bronchopulmonary segments (Fig. 3.47B). Within each bronchopulmonary segment, the segmental bronchi give rise to multiple generations of divisions and, ultimately, to bronchioles, which further subdivide and supply the respiratory surfaces. The walls of the bronchi are held open by discontinuous elongated plates of cartilage, but these are not present in bronchioles. A bronchopulmonary segment is the area of lung supplied by a segmental bronchus and its accompanying pulmonary artery branch. Tributaries of the pulmonary vein tend to pass intersegmentally between and around the margins of segments. Each bronchopulmonary segment is shaped like an irregular cone, with the apex at the origin of the segmental bronchus and the base projected peripherally onto the surface of the lung.


Anatomy_Gray

Anatomy_Gray. Each bronchopulmonary segment is shaped like an irregular cone, with the apex at the origin of the segmental bronchus and the base projected peripherally onto the surface of the lung. A bronchopulmonary segment is the smallest functionally independent region of a lung and the smallest area of lung that can be isolated and removed without affecting adjacent regions. There are ten bronchopulmonary segments in each lung (Fig. 3.48); some of them fuse in the left lung. The right and left pulmonary arteries originate from the pulmonary trunk and carry deoxygenated blood to the lungs from the right ventricle of the heart (Fig. 3.49). The bifurcation of the pulmonary trunk occurs to the left of the midline just inferior to vertebral level TIV/V, and anteroinferiorly to the left of the bifurcation of the trachea. The right pulmonary artery is longer than the left and passes horizontally across the mediastinum (Fig. 3.49).


Anatomy_Gray

Anatomy_Gray. The right pulmonary artery is longer than the left and passes horizontally across the mediastinum (Fig. 3.49). It passes: anteriorly and slightly inferiorly to the tracheal bifurcation and anteriorly to the right main bronchus, and posteriorly to the ascending aorta, superior vena cava, and upper right pulmonary vein. The right pulmonary artery enters the root of the lung and gives off a large branch to the superior lobe of the lung. The main vessel continues through the hilum of the lung, gives off a second (recurrent) branch to the superior lobe, and then divides to supply the middle and inferior lobes. The left pulmonary artery is shorter than the right and lies anterior to the descending aorta and posterior to the superior pulmonary vein (Fig. 3.49). It passes through the root and hilum and branches within the lung.


Anatomy_Gray

Anatomy_Gray. On each side a superior pulmonary vein and an inferior pulmonary vein carry oxygenated blood from the lungs back to the heart (Fig. 3.49). The veins begin at the hilum of the lung, pass through the root of the lung, and immediately drain into the left atrium. The bronchial arteries (Fig. 3.49) and veins constitute the “nutritive” vascular system of the pulmonary tissues (bronchial walls and glands, walls of large vessels, and visceral pleura). They interconnect within the lung with branches of the pulmonary arteries and veins. The bronchial arteries originate from the thoracic aorta or one of its branches: A single right bronchial artery normally arises from the third posterior intercostal artery (but occasionally, it originates from the upper left bronchial artery). Two left bronchial arteries arise directly from the anterior surface of the thoracic aorta—the superior left bronchial artery arises at vertebral level TV, and the inferior one inferior to the left bronchus.


Anatomy_Gray

Anatomy_Gray. The bronchial arteries run on the posterior surfaces of the bronchi and ramify in the lungs to supply pulmonary tissues. The bronchial veins drain into: either the pulmonary veins or the left atrium, and into the azygos vein on the right or into the superior intercostal vein or hemiazygos vein on the left. Structures of the lung and the visceral pleura are supplied by visceral afferents and efferents distributed through the anterior pulmonary plexus and posterior pulmonary plexus (Fig. 3.50). These interconnected plexuses lie anteriorly and posteriorly to the tracheal bifurcation and main bronchi. The anterior plexus is much smaller than the posterior plexus. Branches of these plexuses, which ultimately originate from the sympathetic trunks and vagus nerves, are distributed along branches of the airway and vessels. Visceral efferents from: the vagus nerves constrict the bronchioles; the sympathetic system dilates the bronchioles.


Anatomy_Gray

Anatomy_Gray. Visceral efferents from: the vagus nerves constrict the bronchioles; the sympathetic system dilates the bronchioles. Superficial, or subpleural, and deep lymphatics of the lung drain into lymph nodes called tracheobronchial nodes around the roots of lobar and main bronchi and along the sides of the trachea (Fig. 3.51). As a group, these lymph nodes extend from within the lung, through the hilum and root, and into the posterior mediastinum. Efferent vessels from these nodes pass superiorly along the trachea to unite with similar vessels from parasternal nodes and brachiocephalic nodes, which are anterior to brachiocephalic veins in the superior mediastinum, to form the right and left bronchomediastinal trunks. These trunks drain directly into deep veins at the base of the neck, or may drain into the right lymphatic trunk or thoracic duct.


Anatomy_Gray

Anatomy_Gray. The mediastinum is a broad central partition that separates the two laterally placed pleural cavities (Fig. 3.55). It extends: from the sternum to the bodies of the vertebrae, and from the superior thoracic aperture to the diaphragm (Fig. 3.56). The mediastinum contains the thymus gland, the pericardial sac, the heart, the trachea, and the major arteries and veins. Additionally, the mediastinum serves as a passageway for structures such as the esophagus, thoracic duct, and various components of the nervous system as they traverse the thorax on their way to the abdomen.


Anatomy_Gray

Anatomy_Gray. For organizational purposes, the mediastinum is subdivided into several smaller regions. A transverse plane extending from the sternal angle (the junction between the manubrium and the body of the sternum) to the intervertebral disc between vertebrae TIV and TV separates the mediastinum into the: superior mediastinum, and inferior mediastinum, which is further partitioned into the anterior, middle, and posterior mediastinum by the pericardial sac. The area anterior to the pericardial sac and posterior to the body of the sternum is the anterior mediastinum. The region posterior to the pericardial sac and the diaphragm and anterior to the bodies of the vertebrae is the posterior mediastinum. The area in the middle, which includes the pericardial sac and its contents, is the middle mediastinum (Fig. 3.57). The anterior mediastinum is posterior to the body of the sternum and anterior to the pericardial sac (see Fig. 3.57).


Anatomy_Gray

Anatomy_Gray. The anterior mediastinum is posterior to the body of the sternum and anterior to the pericardial sac (see Fig. 3.57). Its superior boundary is a transverse plane passing from the sternal angle to the intervertebral disc between vertebra TIV and TV, separating it from the superior mediastinum. Its inferior boundary is the diaphragm. Laterally, it is bordered by the mediastinal part of parietal pleura on either side. The major structure in the anterior mediastinum is an inferior extension of the thymus gland (Fig. 3.58). Also present are fat, connective tissue, lymph nodes, mediastinal branches of the internal thoracic vessels, and sternopericardial ligaments, which pass from the posterior surface of the body of the sternum to the fibrous pericardium. The middle mediastinum is centrally located in the thoracic cavity. It contains the pericardium, heart, origins of the great vessels, various nerves, and smaller vessels.


Anatomy_Gray

Anatomy_Gray. The middle mediastinum is centrally located in the thoracic cavity. It contains the pericardium, heart, origins of the great vessels, various nerves, and smaller vessels. The pericardium is a fibroserous sac surrounding the heart and the roots of the great vessels. It consists of two components, the fibrous pericardium and the serous pericardium (Fig. 3.59). The fibrous pericardium is a tough connective tissue outer layer that defines the boundaries of the middle mediastinum. The serous pericardium is thin and consists of two parts: The parietal layer of serous pericardium lines the inner surface of the fibrous pericardium. The visceral layer (epicardium) of serous pericardium adheres to the heart and forms its outer covering.


Anatomy_Gray

Anatomy_Gray. The parietal layer of serous pericardium lines the inner surface of the fibrous pericardium. The visceral layer (epicardium) of serous pericardium adheres to the heart and forms its outer covering. The parietal and visceral layers of serous pericardium are continuous at the roots of the great vessels. The narrow space created between the two layers of serous pericardium, containing a small amount of fluid, is the pericardial cavity. This potential space allows for the relatively uninhibited movement of the heart.


Anatomy_Gray

Anatomy_Gray. The fibrous pericardium is a cone-shaped bag with its base on the diaphragm and its apex continuous with the adventitia of the great vessels (Fig. 3.59). The base is attached to the central tendon of the diaphragm and to a small muscular area of the diaphragm on the left side. Anteriorly, it is attached to the posterior surface of the sternum by sternopericardial ligaments. These attachments help to retain the heart in its position in the thoracic cavity. The sac also limits cardiac distention.


Anatomy_Gray

Anatomy_Gray. The phrenic nerves, which innervate the diaphragm and originate from spinal cord levels C3 to C5, pass through the fibrous pericardium and innervate the fibrous pericardium as they travel from their point of origin to their final destination (Fig. 3.60). Their location, within the fibrous pericardium, is directly related to the embryological origin of the diaphragm and the changes that occur during the formation of the pericardial cavity. Similarly, the pericardiacophrenic vessels are also located within and supply the fibrous pericardium as they pass through the thoracic cavity. The parietal layer of serous pericardium is continuous with the visceral layer of serous pericardium around the roots of the great vessels. These reflections of serous pericardium (Fig. 3.61) occur in two locations: one superiorly, surrounding the arteries—the aorta and the pulmonary trunk; the second more posteriorly, surrounding the veins—the superior and inferior vena cava and the pulmonary veins.


Anatomy_Gray

Anatomy_Gray. The zone of reflection surrounding the veins is J-shaped, and the cul-de-sac formed within the J, posterior to the left atrium, is the oblique pericardial sinus. A passage between the two sites of reflected serous pericardium is the transverse pericardial sinus. This sinus lies posterior to the ascending aorta and the pulmonary trunk, anterior to the superior vena cava, and superior to the left atrium. When the pericardium is opened anteriorly during surgery, a finger placed in the transverse sinus separates arteries from veins. A hand placed under the apex of the heart and moved superiorly slips into the oblique sinus. The pericardium is supplied by branches from the internal thoracic, pericardiacophrenic, musculophrenic, and inferior phrenic arteries, and the thoracic aorta. Veins from the pericardium enter the azygos system of veins and the internal thoracic and superior phrenic veins.


Anatomy_Gray

Anatomy_Gray. Veins from the pericardium enter the azygos system of veins and the internal thoracic and superior phrenic veins. Nerves supplying the pericardium arise from the vagus nerve [X], the sympathetic trunks, and the phrenic nerves. It is important to note that the source of somatic sensation (pain) from the parietal pericardium is carried by somatic afferent fibers in the phrenic nerves. For this reason, “pain” related to a pericardial problem may be referred to the supraclavicular region of the shoulder or lateral neck area dermatomes for spinal cord segments C3, C4, and C5.


Anatomy_Gray

Anatomy_Gray. C3, C4, and C5. The general shape and orientation of the heart are that of a pyramid that has fallen over and is resting on one of its sides. Placed in the thoracic cavity, the apex of this pyramid projects forward, downward, and to the left, whereas the base is opposite the apex and faces in a posterior direction (Fig. 3.63). The sides of the pyramid consist of: a diaphragmatic (inferior) surface on which the pyramid rests, an anterior (sternocostal) surface oriented anteriorly, a right pulmonary surface, and a left pulmonary surface. The base of the heart is quadrilateral and directed posteriorly. It consists of: the left atrium, a small portion of the right atrium, and the proximal parts of the great veins (superior and inferior venae cavae and the pulmonary veins) (Fig. 3.64).


Anatomy_Gray

Anatomy_Gray. Because the great veins enter the base of the heart, with the pulmonary veins entering the right and left sides of the left atrium and the superior and inferior venae cavae at the upper and lower ends of the right atrium, the base of the heart is fixed posteriorly to the pericardial wall, opposite the bodies of vertebrae TV to TVIII (TVI to TIX when standing). The esophagus lies immediately posterior to the base. From the base the heart projects forward, downward, and to the left, ending in the apex. The apex of the heart is formed by the inferolateral part of the left ventricle (Fig. 3.65) and is positioned deep to the left fifth intercostal space, 8 to 9 cm from the midsternal line. Surfaces of the heart The anterior surface faces anteriorly and consists mostly of the right ventricle, with some of the right atrium on the right and some of the left ventricle on the left (Fig. 3.65).


Anatomy_Gray

Anatomy_Gray. The anterior surface faces anteriorly and consists mostly of the right ventricle, with some of the right atrium on the right and some of the left ventricle on the left (Fig. 3.65). The heart in the anatomical position rests on the diaphragmatic surface, which consists of the left ventricle and a small portion of the right ventricle separated by the posterior interventricular groove (Fig. 3.66). This surface faces inferiorly, rests on the diaphragm, is separated from the base of the heart by the coronary sinus, and extends from the base to the apex of the heart. The left pulmonary surface faces the left lung, is broad and convex, and consists of the left ventricle and a portion of the left atrium (Fig. 3.66). The right pulmonary surface faces the right lung, is broad and convex, and consists of the right atrium (Fig. 3.66). Some general descriptions of cardiac orientation refer to right, left, inferior (acute), and obtuse margins:


Anatomy_Gray

Anatomy_Gray. Some general descriptions of cardiac orientation refer to right, left, inferior (acute), and obtuse margins: The right and left margins are the same as the right and left pulmonary surfaces of the heart. The inferior margin is defined as the sharp edge between the anterior and diaphragmatic surfaces of the heart (Figs 3.63 and 3.65)—it is formed mostly by the right ventricle and a small portion of the left ventricle near the apex. The obtuse margin separates the anterior and left pulmonary surfaces (Fig. 3.63)—it is round and extends from the left auricle to the cardiac apex (Fig. 3.65), and is formed mostly by the left ventricle and superiorly by a small portion of the left auricle.


Anatomy_Gray

Anatomy_Gray. For radiological evaluations, a thorough understanding of the structures defining the cardiac borders is critical. The right border in a standard posteroanterior view consists of the superior vena cava, the right atrium, and the inferior vena cava (Fig. 3.67A). The left border in a similar view consists of the arch of the aorta, the pulmonary trunk, left auricle, and the left ventricle. The inferior border in this radiological study consists of the right ventricle and the left ventricle at the apex. In lateral views, the right ventricle is seen anteriorly, and the left atrium is visualized posteriorly (Fig. 3.67B). Internal partitions divide the heart into four chambers (i.e., two atria and two ventricles) and produce surface or external grooves referred to as sulci.


Anatomy_Gray

Anatomy_Gray. Internal partitions divide the heart into four chambers (i.e., two atria and two ventricles) and produce surface or external grooves referred to as sulci. The coronary sulcus circles the heart, separating the atria from the ventricles (Fig. 3.68). As it circles the heart, it contains the right coronary artery, the small cardiac vein, the coronary sinus, and the circumflex branch of the left coronary artery. The anterior and posterior interventricular sulci separate the two ventricles—the anterior interventricular sulcus is on the anterior surface of the heart and contains the anterior interventricular artery and the great cardiac vein, and the posterior interventricular sulcus is on the diaphragmatic surface of the heart and contains the posterior interventricular artery and the middle cardiac vein. These sulci are continuous inferiorly, just to the right of the apex of the heart.


Anatomy_Gray

Anatomy_Gray. These sulci are continuous inferiorly, just to the right of the apex of the heart. The heart functionally consists of two pumps separated by a partition (Fig. 3.69A). The right pump receives deoxygenated blood from the body and sends it to the lungs. The left pump receives oxygenated blood from the lungs and sends it to the body. Each pump consists of an atrium and a ventricle separated by a valve. The thin-walled atria receive blood coming into the heart, whereas the relatively thick-walled ventricles pump blood out of the heart. More force is required to pump blood through the body than through the lungs, so the muscular wall of the left ventricle is thicker than the right. Interatrial, interventricular, and atrioventricular septa separate the four chambers of the heart (Fig. 3.69B). The internal anatomy of each chamber is critical to its function.


Anatomy_Gray

Anatomy_Gray. Interatrial, interventricular, and atrioventricular septa separate the four chambers of the heart (Fig. 3.69B). The internal anatomy of each chamber is critical to its function. In the anatomical position, the right border of the heart is formed by the right atrium. This chamber also contributes to the right portion of the heart’s anterior surface. Blood returning to the right atrium enters through one of three vessels. These are: the superior and inferior venae cavae, which together deliver blood to the heart from the body; and the coronary sinus, which returns blood from the walls of the heart itself. The superior vena cava enters the upper posterior portion of the right atrium, and the inferior vena cava and coronary sinus enter the lower posterior portion of the right atrium.


Anatomy_Gray

Anatomy_Gray. The superior vena cava enters the upper posterior portion of the right atrium, and the inferior vena cava and coronary sinus enter the lower posterior portion of the right atrium. From the right atrium, blood passes into the right ventricle through the right atrioventricular orifice. This opening faces forward and medially and is closed during ventricular contraction by the tricuspid valve. The interior of the right atrium is divided into two continuous spaces. Externally, this separation is indicated by a shallow, vertical groove (the sulcus terminalis cordis), which extends from the right side of the opening of the superior vena cava to the right side of the opening of the inferior vena cava. Internally, this division is indicated by the crista terminalis (Fig. 3.70), which is a smooth, muscular ridge that begins on the roof of the atrium just in front of the opening of the superior vena cava and extends down the lateral wall to the anterior lip of the inferior vena cava.


Anatomy_Gray

Anatomy_Gray. The space posterior to the crista is the sinus of venae cavae and is derived embryologically from the right horn of the sinus venosus. This component of the right atrium has smooth, thin walls, and both venae cavae empty into this space. The space anterior to the crista, including the right auricle, is sometimes referred to as the atrium proper. This terminology is based on its origin from the embryonic primitive atrium. Its walls are covered by ridges called the musculi pectinati (pectinate muscles), which fan out from the crista like the “teeth of a comb.” These ridges are also found in the right auricle, which is an ear-like, conical, muscular pouch that externally overlaps the ascending aorta.


Anatomy_Gray

Anatomy_Gray. An additional structure in the right atrium is the opening of the coronary sinus, which receives blood from most of the cardiac veins and opens medially to the opening of the inferior vena cava. Associated with these openings are small folds of tissue derived from the valve of the embryonic sinus venosus (the valve of the coronary sinus and the valve of inferior vena cava, respectively). During development, the valve of the inferior vena cava helps direct incoming oxygenated blood through the foramen ovale and into the left atrium. Separating the right atrium from the left atrium is the interatrial septum, which faces forward and to the right because the left atrium lies posteriorly and to the left of the right atrium. A depression is clearly visible in the septum just above the orifice of the inferior vena cava. This is the fossa ovalis (oval fossa), with its prominent margin, the limbus fossa ovalis (border of the oval fossa).


Anatomy_Gray

Anatomy_Gray. The fossa ovalis marks the location of the embryonic foramen ovale, which is an important part of fetal circulation. The foramen ovale allows oxygenated blood entering the right atrium through the inferior vena cava to pass directly to the left atrium and so bypass the lungs, which are nonfunctional before birth. Finally, numerous small openings—the openings of the smallest cardiac veins (the foramina of the venae cordis minimae)—are scattered along the walls of the right atrium. These are small veins that drain the myocardium directly into the right atrium. In the anatomical position, the right ventricle forms most of the anterior surface of the heart and a portion of the diaphragmatic surface. The right atrium is to the right of the right ventricle and the right ventricle is located in front of and to the left of the right atrioventricular orifice. Blood entering the right ventricle from the right atrium therefore moves in a horizontal and forward direction.


Anatomy_Gray

Anatomy_Gray. The outflow tract of the right ventricle, which leads to the pulmonary trunk, is the conus arteriosus (infundibulum). This area has smooth walls and derives from the embryonic bulbus cordis. The walls of the inflow portion of the right ventricle have numerous muscular, irregular structures called trabeculae carneae (Fig. 3.71). Most of these are either attached to the ventricular walls throughout their length, forming ridges, or attached at both ends, forming bridges. A few trabeculae carneae (papillary muscles) have only one end attached to the ventricular surface, while the other end serves as the point of attachment for tendon-like fibrous cords (the chordae tendineae), which connect to the free edges of the cusps of the tricuspid valve. There are three papillary muscles in the right ventricle. Named relative to their point of origin on the ventricular surface, they are the anterior, posterior, and septal papillary muscles:


Anatomy_Gray

Anatomy_Gray. There are three papillary muscles in the right ventricle. Named relative to their point of origin on the ventricular surface, they are the anterior, posterior, and septal papillary muscles: The anterior papillary muscle is the largest and most constant papillary muscle, and arises from the anterior wall of the ventricle. The posterior papillary muscle may consist of one, two, or three structures, with some chordae tendineae arising directly from the ventricular wall. The septal papillary muscle is the most inconsistent papillary muscle, being either small or absent, with chordae tendineae emerging directly from the septal wall.


Anatomy_Gray

Anatomy_Gray. The septal papillary muscle is the most inconsistent papillary muscle, being either small or absent, with chordae tendineae emerging directly from the septal wall. A single specialized trabeculum, the septomarginal trabecula (moderator band), forms a bridge between the lower portion of the interventricular septum and the base of the anterior papillary muscle. The septomarginal trabecula carries a portion of the cardiac conduction system, the right bundle of the atrioventricular bundle, to the anterior wall of the right ventricle. The right atrioventricular orifice is closed during ventricular contraction by the tricuspid valve (right atrioventricular valve), so named because it usually consists of three cusps or leaflets (Fig. 3.71). The base of each cusp is secured to the fibrous ring that surrounds the atrioventricular orifice. This fibrous ring helps to maintain the shape of the opening. The cusps are continuous with each other near their bases at sites termed commissures.


Anatomy_Gray

Anatomy_Gray. The naming of the three cusps, the anterior, septal, and posterior cusps, is based on their relative position in the right ventricle. The free margins of the cusps are attached to the chordae tendineae, which arise from the tips of the papillary muscles. During filling of the right ventricle, the tricuspid valve is open, and the three cusps project into the right ventricle. Without the presence of a compensating mechanism, when the ventricular musculature contracts, the valve cusps would be forced upward with the flow of blood and blood would move back into the right atrium. However, contraction of the papillary muscles attached to the cusps by chordae tendineae prevents the cusps from being everted into the right atrium. Simply put, the papillary muscles and associated chordae tendineae keep the valves closed during the dramatic changes in ventricular size that occur during contraction.


Anatomy_Gray

Anatomy_Gray. Simply put, the papillary muscles and associated chordae tendineae keep the valves closed during the dramatic changes in ventricular size that occur during contraction. In addition, chordae tendineae from two papillary muscles attach to each cusp. This helps prevent separation of the cusps during ventricular contraction. Proper closing of the tricuspid valve causes blood to exit the right ventricle and move into the pulmonary trunk. Necrosis of a papillary muscle following a myocardial infarction (heart attack) may result in prolapse of the related valve.


Anatomy_Gray

Anatomy_Gray. Necrosis of a papillary muscle following a myocardial infarction (heart attack) may result in prolapse of the related valve. At the apex of the infundibulum, the outflow tract of the right ventricle, the opening into the pulmonary trunk is closed by the pulmonary valve (Fig. 3.71), which consists of three semilunar cusps with free edges projecting upward into the lumen of the pulmonary trunk. The free superior edge of each cusp has a middle, thickened portion, the nodule of the semilunar cusp, and a thin lateral portion, the lunula of the semilunar cusp (Fig. 3.72).


Anatomy_Gray

Anatomy_Gray. The cusps are named the left, right, and anterior semilunar cusps, relative to their fetal position before rotation of the outflow tracts from the ventricles is complete. Each cusp forms a pocket-like sinus (Fig. 3.72)—a dilation in the wall of the initial portion of the pulmonary trunk. After ventricular contraction, the recoil of blood fills these pulmonary sinuses and forces the cusps closed. This prevents blood in the pulmonary trunk from refilling the right ventricle. The left atrium forms most of the base or posterior surface of the heart. As with the right atrium, the left atrium is derived embryologically from two structures. The posterior half, or inflow portion, receives the four pulmonary veins (Fig. 3.73). It has smooth walls and derives from the proximal parts of the pulmonary veins that are incorporated into the left atrium during development.


Anatomy_Gray

Anatomy_Gray. The anterior half is continuous with the left auricle. It contains musculi pectinati and derives from the embryonic primitive atrium. Unlike the crista terminalis in the right atrium, no distinct structure separates the two components of the left atrium. The interatrial septum is part of the anterior wall of the left atrium. The thin area or depression in the septum is the valve of the foramen ovale and is opposite the floor of the fossa ovalis in the right atrium. During development, the valve of the foramen ovale prevents blood from passing from the left atrium to the right atrium. This valve may not be completely fused in some adults, leaving a “probe patent” passage between the right atrium and the left atrium. The left ventricle lies anterior to the left atrium. It contributes to the anterior, diaphragmatic, and left pulmonary surfaces of the heart, and forms the apex.


Anatomy_Gray

Anatomy_Gray. The left ventricle lies anterior to the left atrium. It contributes to the anterior, diaphragmatic, and left pulmonary surfaces of the heart, and forms the apex. Blood enters the ventricle through the left atrioventricular orifice and flows in a forward direction to the apex. The chamber itself is conical, is longer than the right ventricle, and has the thickest layer of myocardium. The outflow tract (the aortic vestibule) is posterior to the infundibulum of the right ventricle, has smooth walls, and is derived from the embryonic bulbus cordis. The trabeculae carneae in the left ventricle are fine and delicate in contrast to those in the right ventricle. The general appearance of the trabeculae with muscular ridges and bridges is similar to that of the right ventricle (Fig. 3.74).


Anatomy_Gray

Anatomy_Gray. The general appearance of the trabeculae with muscular ridges and bridges is similar to that of the right ventricle (Fig. 3.74). Papillary muscles, together with chordae tendineae, are also observed and their structure is as described above for the right ventricle. Two papillary muscles, the anterior and posterior papillary muscles, are usually found in the left ventricle and are larger than those of the right ventricle. In the anatomical position, the left ventricle is somewhat posterior to the right ventricle. The interventricular septum therefore forms the anterior wall and some of the wall on the right side of the left ventricle. The septum is described as having two parts: a muscular part, and a membranous part.


Anatomy_Gray

Anatomy_Gray. The muscular part is thick and forms the major part of the septum, whereas the membranous part is the thin, upper part of the septum. A third part of the septum may be considered an atrioventricular part because of its position above the septal cusp of the tricuspid valve. This superior location places this part of the septum between the left ventricle and right atrium. The left atrioventricular orifice opens into the posterior right side of the superior part of the left ventricle. It is closed during ventricular contraction by the mitral valve (left atrioventricular valve), which is also referred to as the bicuspid valve because it has two cusps, the anterior and posterior cusps (Fig. 3.74). The bases of the cusps are secured to a fibrous ring surrounding the opening, and the cusps are continuous with each other at the commissures. The coordinated action of the papillary muscles and chordae tendineae is as described for the right ventricle.


Anatomy_Gray

Anatomy_Gray. The aortic vestibule, or outflow tract of the left ventricle, is continuous superiorly with the ascending aorta. The opening from the left ventricle into the aorta is closed by the aortic valve. This valve is similar in structure to the pulmonary valve. It consists of three semilunar cusps with the free edge of each projecting upward into the lumen of the ascending aorta (Fig. 3.75). Between the semilunar cusps and the wall of the ascending aorta are pocket-like sinuses—the right, left, and posterior aortic sinuses. The right and left coronary arteries originate from the right and left aortic sinuses. Because of this, the posterior aortic sinus and cusp are sometimes referred to as the noncoronary sinus and cusp.


Anatomy_Gray

Anatomy_Gray. The functioning of the aortic valve is similar to that of the pulmonary valve with one important additional process: as blood recoils after ventricular contraction and fills the aortic sinuses, it is automatically forced into the coronary arteries because these vessels originate from the right and left aortic sinuses. The cardiac skeleton is a collection of dense, fibrous connective tissue in the form of four rings with interconnecting areas in a plane between the atria and the ventricles. The four rings of the cardiac skeleton surround the two atrioventricular orifices, the aortic orifice and opening of the pulmonary trunks. They are the anulus fibrosus. The interconnecting areas include: the right fibrous trigone, which is a thickened area of connective tissue between the aortic ring and right atrioventricular ring; and the left fibrous trigone, which is a thickened area of connective tissue between the aortic ring and the left atrioventricular ring (Fig. 3.76).


Anatomy_Gray

Anatomy_Gray. The cardiac skeleton helps maintain the integrity of the openings it surrounds and provides points of attachment for the cusps. It also separates the atrial musculature from the ventricular musculature. The atrial myocardium originates from the upper border of the rings, whereas the ventricular myocardium originates from the lower border of the rings. The cardiac skeleton also serves as a dense connective tissue partition that electrically isolates the atria from the ventricles. The atrioventricular bundle, which passes through the anulus, is the single connection between these two groups of myocardium. Two coronary arteries arise from the aortic sinuses in the initial portion of the ascending aorta and supply the muscle and other tissues of the heart. They circle the heart in the coronary sulcus, with marginal and interventricular branches, in the interventricular sulci, converging toward the apex of the heart (Fig. 3.77).


Anatomy_Gray

Anatomy_Gray. The returning venous blood passes through cardiac veins, most of which empty into the coronary sinus. This large venous structure is located in the coronary sulcus on the posterior surface of the heart between the left atrium and left ventricle. The coronary sinus empties into the right atrium between the opening of the inferior vena cava and the right atrioventricular orifice. Right coronary artery. The right coronary artery originates from the right aortic sinus of the ascending aorta. It passes anteriorly and then descends vertically in the coronary sulcus, between the right atrium and right ventricle (Fig. 3.78A). On reaching the inferior margin of the heart, it turns posteriorly and continues in the sulcus onto the diaphragmatic surface and base of the heart. During this course, several branches arise from the main stem of the vessel:


Anatomy_Gray

Anatomy_Gray. An early atrial branch passes in the groove between the right auricle and ascending aorta, and gives off the sinu-atrial nodal branch (Fig. 3.78A), which passes posteriorly around the superior vena cava to supply the sinu-atrial node. A right marginal branch is given off as the right coronary artery approaches the inferior (acute) margin of the heart (Fig. 3.78A,B) and continues along this border toward the apex of the heart. As the right coronary artery continues on the base/ diaphragmatic surface of the heart, it supplies a small branch to the atrioventricular node before giving off its final major branch, the posterior interventricular branch (Fig. 3.78A), which lies in the posterior interventricular sulcus.


Anatomy_Gray

Anatomy_Gray. The right coronary artery supplies the right atrium and right ventricle, the sinu-atrial and atrioventricular nodes, the interatrial septum, a portion of the left atrium, the posteroinferior one third of the interventricular septum, and a portion of the posterior part of the left ventricle. Left coronary artery. The left coronary artery originates from the left aortic sinus of the ascending aorta. It passes between the pulmonary trunk and the left auricle before entering the coronary sulcus. Emerging from behind the pulmonary trunk, the artery divides into its two terminal branches, the anterior interventricular and the circumflex (Fig. 3.78A).


Anatomy_Gray

Anatomy_Gray. The anterior interventricular branch (left anterior descending artery—LAD) (Fig. 3.78A,C) continues around the left side of the pulmonary trunk and descends obliquely toward the apex of the heart in the anterior interventricular sulcus (Fig. 3.78A,C). During its course, one or two large diagonal branches may arise and descend diagonally across the anterior surface of the left ventricle. The circumflex branch (Fig. 3.78A,C) courses toward the left, in the coronary sulcus and onto the base/diaphragmatic surface of the heart, and usually ends before reaching the posterior interventricular sulcus. A large branch, the left marginal artery (Fig. 3.78A,C), usually arises from it and continues across the rounded obtuse margin of the heart. The distribution pattern of the left coronary artery enables it to supply most of the left atrium and left ventricle, and most of the interventricular septum, including the atrioventricular bundle and its branches.


Anatomy_Gray

Anatomy_Gray. Variations in the distribution patterns of coronary arteries. Several major variations in the basic distribution patterns of the coronary arteries occur. The distribution pattern described above for both right and left coronary arteries is the most common and consists of a right dominant coronary artery. This means that the posterior interventricular branch arises from the right coronary artery. The right coronary artery therefore supplies a large portion of the posterior wall of the left ventricle and the circumflex branch of the left coronary artery is relatively small. In contrast, in hearts with a left dominant coronary artery, the posterior interventricular branch arises from an enlarged circumflex branch and supplies most of the posterior wall of the left ventricle (Fig. 3.79).


Anatomy_Gray

Anatomy_Gray. Another point of variation relates to the arterial supply to the sinu-atrial and atrioventricular nodes. In most cases, these two structures are supplied by the right coronary artery. However, vessels from the circumflex branch of the left coronary artery occasionally supply these structures. The coronary sinus receives four major tributaries: the great, middle, small, and posterior cardiac veins.


Anatomy_Gray

Anatomy_Gray. The coronary sinus receives four major tributaries: the great, middle, small, and posterior cardiac veins. Great cardiac vein. The great cardiac vein begins at the apex of the heart (Fig. 3.82A). It ascends in the anterior interventricular sulcus, where it is related to the anterior interventricular artery and is often termed the anterior interventricular vein. Reaching the coronary sulcus, the great cardiac vein turns to the left and continues onto the base/diaphragmatic surface of the heart. At this point, it is associated with the circumflex branch of the left coronary artery. Continuing along its path in the coronary sulcus, the great cardiac vein gradually enlarges to form the coronary sinus, which enters the right atrium (Fig. 3.82B).


Anatomy_Gray

Anatomy_Gray. Middle cardiac vein. The middle cardiac vein (posterior interventricular vein) begins near the apex of the heart and ascends in the posterior interventricular sulcus toward the coronary sinus (Fig. 3.82B). It is associated with the posterior interventricular branch of the right or left coronary artery throughout its course. Small cardiac vein. The small cardiac vein begins in the lower anterior section of the coronary sulcus between the right atrium and right ventricle (Fig. 3.82A). It continues in this groove onto the base/diaphragmatic surface of the heart where it enters the coronary sinus at its atrial end. It is a companion of the right coronary artery throughout its course and may receive the right marginal vein (Fig. 3.82A). This small vein accompanies the marginal branch of the right coronary artery along the acute margin of the heart. If the right marginal vein does not join the small cardiac vein, it enters the right atrium directly.


Anatomy_Gray

Anatomy_Gray. Posterior cardiac vein. The posterior cardiac vein lies on the posterior surface of the left ventricle just to the left of the middle cardiac vein (Fig. 3.82B). It either enters the coronary sinus directly or joins the great cardiac vein. Other cardiac veins. Two additional groups of cardiac veins are also involved in the venous drainage of the heart. The anterior veins of the right ventricle (anterior cardiac veins) are small veins that arise on the anterior surface of the right ventricle (Fig. 3.82A). They cross the coronary sulcus and enter the anterior wall of the right atrium. They drain the anterior portion of the right ventricle. The right marginal vein may be part of this group if it does not enter the small cardiac vein.


Anatomy_Gray

Anatomy_Gray. A group of smallest cardiac veins (venae cordis minimae or veins of Thebesius) have also been described. Draining directly into the cardiac chambers, they are numerous in the right atrium and right ventricle, are occasionally associated with the left atrium, and are rarely associated with the left ventricle. The lymphatic vessels of the heart follow the coronary arteries and drain mainly into: brachiocephalic nodes, anterior to the brachiocephalic veins; and tracheobronchial nodes, at the inferior end of the trachea. The musculature of the atria and ventricles is capable of contracting spontaneously. The cardiac conduction system initiates and coordinates contraction. The conduction system consists of nodes and networks of specialized cardiac muscle cells organized into four basic components: the sinu-atrial node, the atrioventricular node, the atrioventricular bundle with its right and left bundle branches, and the subendocardial plexus of conduction cells (the Purkinje fibers).


Anatomy_Gray

Anatomy_Gray. The unique distribution pattern of the cardiac conduction system establishes an important unidirectional pathway of excitation/contraction. Throughout its course, large branches of the conduction system are insulated from the surrounding myocardium by connective tissue. This tends to decrease inappropriate stimulation and contraction of cardiac muscle fibers. The number of functional contacts between the conduction pathway and cardiac musculature greatly increases in the subendocardial network. Thus, a unidirectional wave of excitation and contraction is established, which moves from the papillary muscles and apex of the ventricles to the arterial outflow tracts. Impulses begin at the sinu-atrial node, the cardiac pacemaker. This collection of cells is located at the superior end of the crista terminalis at the junction of the superior vena cava and the right atrium (Fig. 3.83A).


Anatomy_Gray

Anatomy_Gray. This is also the junction between the parts of the right atrium derived from the embryonic sinus venosus and the atrium proper. The excitation signals generated by the sinu-atrial node spread across the atria, causing the muscle to contract. Concurrently, the wave of excitation in the atria stimulates the atrioventricular node, which is located near the opening of the coronary sinus, close to the attachment of the septal cusp of the tricuspid valve, and within the atrioventricular septum (Fig. 3.83A). The atrioventricular node is a collection of specialized cells that forms the beginning of an elaborate system of conducting tissue, the atrioventricular bundle, which extends the excitatory impulse to all ventricular musculature. The atrioventricular bundle is a direct continuation of the atrioventricular node (Fig. 3.83A). It follows along the lower border of the membranous part of the interventricular septum before splitting into right and left bundles.


Anatomy_Gray

Anatomy_Gray. The right bundle branch continues on the right side of the interventricular septum toward the apex of the right ventricle. From the septum it enters the septomarginal trabecula to reach the base of the anterior papillary muscle. At this point, it divides and is continuous with the final component of the cardiac conduction system, the subendocardial plexus of ventricular conduction cells or Purkinje fibers. This network of specialized cells spreads throughout the ventricle to supply the ventricular musculature, including the papillary muscles. The left bundle branch passes to the left side of the muscular interventricular septum and descends to the apex of the left ventricle (Fig. 3.83B). Along its course it gives off branches that eventually become continuous with the subendocardial plexus of conduction cells (Purkinje fibers). As with the right side, this network of specialized cells spreads the excitation impulses throughout the left ventricle.


Anatomy_Gray

Anatomy_Gray. The autonomic division of the peripheral nervous system is directly responsible for regulating: heart rate, force of each contraction, and cardiac output. Branches from both the parasympathetic and sympathetic systems contribute to the formation of the cardiac plexus. This plexus consists of a superficial part, inferior to the aortic arch and between it and the pulmonary trunk (Fig. 3.84A), and a deep part, between the aortic arch and the tracheal bifurcation (Fig. 3.84B). From the cardiac plexus, small branches that are mixed nerves containing both sympathetic and parasympathetic fibers supply the heart. These branches affect nodal tissue and other components of the conduction system, coronary blood vessels, and atrial and ventricular musculature. Stimulation of the parasympathetic system: decreases heart rate, reduces force of contraction, and constricts the coronary arteries.


Anatomy_Gray

Anatomy_Gray. Stimulation of the parasympathetic system: decreases heart rate, reduces force of contraction, and constricts the coronary arteries. The preganglionic parasympathetic fibers reach the heart as cardiac branches from the right and left vagus nerves. They enter the cardiac plexus and synapse in ganglia located either within the plexus or in the walls of the atria. Stimulation of the sympathetic system: increases heart rate, and increases the force of contraction. Sympathetic fibers reach the cardiac plexus through the cardiac nerves from the sympathetic trunk. Preganglionic sympathetic fibers from the upper four or five segments of the thoracic spinal cord enter and move through the sympathetic trunk. They synapse in cervical and upper thoracic sympathetic ganglia, and postganglionic fibers proceed as bilateral branches from the sympathetic trunk to the cardiac plexus.


Anatomy_Gray

Anatomy_Gray. Visceral afferents from the heart are also a component of the cardiac plexus. These fibers pass through the cardiac plexus and return to the central nervous system in the cardiac nerves from the sympathetic trunk and in the vagal cardiac branches. The afferents associated with the vagal cardiac nerves return to the vagus nerve [X]. They sense alterations in blood pressure and blood chemistry and are therefore primarily concerned with cardiac reflexes.


Anatomy_Gray

Anatomy_Gray. The afferents associated with the cardiac nerves from the sympathetic trunks return to either the cervical or the thoracic portions of the sympathetic trunk. If they are in the cervical portion of the trunk, they normally descend to the thoracic region, where they reenter the upper four or five thoracic spinal cord segments, along with the afferents from the thoracic region of the sympathetic trunk. Visceral afferents associated with the sympathetic system conduct pain sensation from the heart, which is detected at the cellular level as tissue-damaging events (i.e., cardiac ischemia). This pain is often “referred” to cutaneous regions supplied by the same spinal cord levels (see “In the clinic: Referred pain.” p. 46, and “Case 1,” pp. 244–246).


Anatomy_Gray

Anatomy_Gray. The pulmonary trunk is contained within the pericardial sac (Fig. 3.85), is covered by the visceral layer of serous pericardium, and is associated with the ascending aorta in a common sheath. It arises from the conus arteriosus of the right ventricle at the opening of the pulmonary trunk slightly anterior to the aortic orifice and ascends, moving posteriorly and to the left, lying initially anterior and then to the left of the ascending aorta. At approximately the level of the intervertebral disc between vertebrae TV and TVI, opposite the left border of the sternum and posterior to the third left costal cartilage, the pulmonary trunk divides into: the right pulmonary artery, which passes to the right, posterior to the ascending aorta and the superior vena cava, to enter the right lung; and the left pulmonary artery, which passes inferiorly to the arch of the aorta and anteriorly to the descending aorta to enter the left lung.


Anatomy_Gray

Anatomy_Gray. The ascending aorta is contained within the pericardial sac and is covered by a visceral layer of serous pericardium, which also surrounds the pulmonary trunk in a common sheath (Fig. 3.85A). The origin of the ascending aorta is the aortic orifice at the base of the left ventricle, which is level with the lower edge of the third left costal cartilage, posterior to the left half of the sternum. Moving superiorly, slightly forward and to the right, the ascending aorta continues to the level of the second right costal cartilage. At this point, it enters the superior mediastinum and is then referred to as the arch of the aorta. Immediately superior to the point where the ascending aorta arises from the left ventricle are three small outward bulges opposite the semilunar cusps of the aortic valve. These are the posterior, right, and left aortic sinuses. The right and left coronary arteries originate from the right and left aortic sinuses, respectively.


Anatomy_Gray

Anatomy_Gray. The inferior half of the superior vena cava is located within the pericardial sac (Fig. 3.85B). It passes through the fibrous pericardium at approximately the level of the second costal cartilage and enters the right atrium at the lower level of the third costal cartilage. The portion within the pericardial sac is covered with serous pericardium except for a small area on its posterior surface. After passing through the diaphragm, at approximately the level of vertebra TVIII, the inferior vena cava enters the fibrous pericardium. A short portion of this vessel is within the pericardial sac before entering the right atrium. While within the pericardial sac, it is covered by serous pericardium except for a small portion of its posterior surface (Fig. 3.85B).


Anatomy_Gray

Anatomy_Gray. A very short segment of each of the pulmonary veins is also within the pericardial sac. These veins, usually two from each lung, pass through the fibrous pericardium and enter the superior region of the left atrium on its posterior surface. In the pericardial sac, all but a portion of the posterior surface of these veins is covered by serous pericardium. In addition, the oblique pericardial sinus is between the right and left pulmonary veins, within the pericardial sac (Fig. 3.85B). The superior mediastinum is posterior to the manubrium of the sternum and anterior to the bodies of the first four thoracic vertebrae (see Fig. 3.57). Its superior boundary is an oblique plane passing from the jugular notch upward and posteriorly to the superior border of vertebra TI. Inferiorly, a transverse plane passing from the sternal angle to the intervertebral disc between vertebra TIV/V separates it from the inferior mediastinum.


Anatomy_Gray

Anatomy_Gray. Inferiorly, a transverse plane passing from the sternal angle to the intervertebral disc between vertebra TIV/V separates it from the inferior mediastinum. Laterally, it is bordered by the mediastinal part of the parietal pleura on either side. The superior mediastinum is continuous with the neck above and with the inferior mediastinum below. The major structures found in the superior mediastinum (Figs. 3.86 and 3.87) include the: thymus, right and left brachiocephalic veins, left superior intercostal vein, superior vena cava, arch of the aorta with its three large branches, trachea, esophagus, phrenic nerves, vagus nerves, left recurrent laryngeal branch of the left vagus nerve, thoracic duct, and other small nerves, blood vessels, and lymphatics. The thymus is the most anterior component of the superior mediastinum, lying immediately posterior to the manubrium of the sternum. It is an asymmetrical, bilobed structure (see Fig. 3.58).


Anatomy_Gray

Anatomy_Gray. The thymus is the most anterior component of the superior mediastinum, lying immediately posterior to the manubrium of the sternum. It is an asymmetrical, bilobed structure (see Fig. 3.58). The upper extent of the thymus can reach into the neck as high as the thyroid gland; a lower portion typically extends into the anterior mediastinum over the pericardial sac. Involved in the early development of the immune system, the thymus is a large structure in the child, begins to atrophy after puberty, and shows considerable size variation in the adult. In the elderly adult, it is barely identifiable as an organ, consisting mostly of fatty tissue that is sometimes arranged as two lobulated fatty structures. Arteries to the thymus consist of small branches originating from the internal thoracic arteries. Venous drainage is usually into the left brachiocephalic vein and possibly into the internal thoracic veins.


Anatomy_Gray

Anatomy_Gray. Lymphatic drainage returns to multiple groups of nodes at one or more of the following locations: along the internal thoracic arteries (parasternal); at the tracheal bifurcation (tracheobronchial); and in the root of the neck. The left and right brachiocephalic veins are located immediately posterior to the thymus. They form on each side at the junction between the internal jugular and subclavian veins (see Fig. 3.86). The left brachiocephalic vein crosses the midline and joins with the right brachiocephalic vein to form the superior vena cava (Fig. 3.88). The right brachiocephalic vein begins posterior to the medial end of the right clavicle and passes vertically downward, forming the superior vena cava when it is joined by the left brachiocephalic vein. Venous tributaries include the vertebral, first posterior intercostal, and internal thoracic veins. The inferior thyroid and thymic veins may also drain into it.


Anatomy_Gray

Anatomy_Gray. The left brachiocephalic vein begins posterior to the medial end of the left clavicle. It crosses to the right, moving in a slightly inferior direction, and joins with the right brachiocephalic vein to form the superior vena cava posterior to the lower edge of the right first costal cartilage close to the right sternal border. Venous tributaries include the vertebral, first posterior intercostal, left superior intercostal, inferior thyroid, and internal thoracic veins. It may also receive thymic and pericardial veins. The left brachiocephalic vein crosses the midline posterior to the manubrium in the adult. In infants and children the left brachiocephalic vein rises above the superior border of the manubrium and therefore is less protected.


Anatomy_Gray

Anatomy_Gray. The left superior intercostal vein receives the second, third, and sometimes the fourth posterior intercostal veins, usually the left bronchial veins, and sometimes the left pericardiacophrenic vein. It passes over the left side of the aortic arch, lateral to the left vagus nerve and medial to the left phrenic nerve, before entering the left brachiocephalic vein (Fig. 3.89). Inferiorly, it may connect with the accessory hemiazygos vein (superior hemiazygos vein). The vertically oriented superior vena cava begins posterior to the lower edge of the right first costal cartilage, where the right and left brachiocephalic veins join, and terminates at the lower edge of the right third costal cartilage, where it joins the right atrium (see Fig. 3.86). The lower half of the superior vena cava is within the pericardial sac and is therefore contained in the middle mediastinum.


Anatomy_Gray

Anatomy_Gray. The lower half of the superior vena cava is within the pericardial sac and is therefore contained in the middle mediastinum. The superior vena cava receives the azygos vein immediately before entering the pericardial sac and may also receive pericardial and mediastinal veins. The superior vena cava can be easily visualized forming part of the right superolateral border of the mediastinum on a chest radiograph (see Fig. 3.67A). Arch of aorta and its branches


Anatomy_Gray

Anatomy_Gray. The superior vena cava can be easily visualized forming part of the right superolateral border of the mediastinum on a chest radiograph (see Fig. 3.67A). Arch of aorta and its branches The thoracic portion of the aorta can be divided into ascending aorta, arch of the aorta, and thoracic (descending) aorta. Only the arch of the aorta is in the superior mediastinum. It begins when the ascending aorta emerges from the pericardial sac and courses upward, backward, and to the left as it passes through the superior mediastinum, ending on the left side at vertebral level TIV/V (see Fig. 3.86). Extending as high as the midlevel of the manubrium of the sternum, the arch is initially anterior and finally lateral to the trachea. Three branches arise from the superior border of the arch of the aorta; at their origins, all three are crossed anteriorly by the left brachiocephalic vein. The first branch


Anatomy_Gray

Anatomy_Gray. Three branches arise from the superior border of the arch of the aorta; at their origins, all three are crossed anteriorly by the left brachiocephalic vein. The first branch Beginning on the right, the first branch of the arch of the aorta is the brachiocephalic trunk (Fig. 3.90). It is the largest of the three branches and, at its point of origin behind the manubrium of the sternum, is slightly anterior to the other two branches. It ascends slightly posteriorly and to the right. At the level of the upper edge of the right sternoclavicular joint, the brachiocephalic trunk divides into: the right common carotid artery, and the right subclavian artery (see Fig. 3.86). The arteries mainly supply the right side of the head and neck and the right upper limb, respectively. Occasionally, the brachiocephalic trunk has a small branch, the thyroid ima artery, which contributes to the vascular supply of the thyroid gland. The second branch


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Anatomy_Gray. Occasionally, the brachiocephalic trunk has a small branch, the thyroid ima artery, which contributes to the vascular supply of the thyroid gland. The second branch The second branch of the arch of the aorta is the left common carotid artery (Fig. 3.90). It arises from the arch immediately to the left and slightly posterior to the brachiocephalic trunk and ascends through the superior mediastinum along the left side of the trachea. The left common carotid artery supplies the left side of the head and neck. The third branch The third branch of the arch of the aorta is the left subclavian artery (Fig. 3.90). It arises from the arch of the aorta immediately to the left of, and slightly posterior to, the left common carotid artery and ascends through the superior mediastinum along the left side of the trachea. The left subclavian artery is the major blood supply to the left upper limb.


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Anatomy_Gray. The left subclavian artery is the major blood supply to the left upper limb. The ligamentum arteriosum is also in the superior mediastinum and is important in embryonic circulation, when it is a patent vessel (the ductus arteriosus). It connects the pulmonary trunk with the arch of the aorta and allows blood to bypass the lungs during development (Fig. 3.90). The vessel closes soon after birth and forms the ligamentous connection observed in the adult. The trachea is a midline structure that is palpable in the jugular notch as it enters the superior mediastinum. Posterior to it is the esophagus, which is immediately anterior to the vertebral column (Fig. 3.92, and see Figs. 3.86 and 3.87). Significant mobility exists in the vertical positioning of these structures as they pass through the superior mediastinum. Swallowing and breathing cause positional shifts, as may disease and the use of specialized instrumentation.


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Anatomy_Gray. As the trachea and esophagus pass through the superior mediastinum, they are crossed laterally by the azygos vein on the right side and the arch of the aorta on the left side. The trachea divides into the right and left main bronchi at, or just inferior to, the transverse plane between the sternal angle and vertebral level TIV/V (Fig. 3.93), whereas the esophagus continues into the posterior mediastinum. Nerves of the superior mediastinum The vagus nerves [X] pass through the superior and posterior divisions of the mediastinum on their way to the abdominal cavity. As they pass through the thorax, they provide parasympathetic innervation to the thoracic viscera and carry visceral afferents from the thoracic viscera. Visceral afferents in the vagus nerves relay information to the central nervous system about normal physiological processes and reflex activities. They do not transmit pain sensation.


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Anatomy_Gray. Visceral afferents in the vagus nerves relay information to the central nervous system about normal physiological processes and reflex activities. They do not transmit pain sensation. The right vagus nerve enters the superior mediastinum and lies between the right brachiocephalic vein and the brachiocephalic trunk. It descends in a posterior direction toward the trachea (Fig. 3.94), crosses the lateral surface of the trachea, and passes posteriorly to the root of the right lung to reach the esophagus. Just before the esophagus, it is crossed by the arch of the azygos vein. As the right vagus nerve passes through the superior mediastinum, it gives branches to the esophagus, cardiac plexus, and pulmonary plexus.


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Anatomy_Gray. As the right vagus nerve passes through the superior mediastinum, it gives branches to the esophagus, cardiac plexus, and pulmonary plexus. The left vagus nerve enters the superior mediastinum posterior to the left brachiocephalic vein and between the left common carotid and left subclavian arteries (Fig. 3.95). As it passes into the superior mediastinum, it lies just deep to the mediastinal part of the parietal pleura and crosses the left side of the arch of the aorta. It continues to descend in a posterior direction and passes posterior to the root of the left lung to reach the esophagus in the posterior mediastinum. As the left vagus nerve passes through the superior mediastinum, it gives branches to the esophagus, the cardiac plexus, and the pulmonary plexus.


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Anatomy_Gray. As the left vagus nerve passes through the superior mediastinum, it gives branches to the esophagus, the cardiac plexus, and the pulmonary plexus. The left vagus nerve also gives rise to the left recurrent laryngeal nerve, which arises from it at the inferior margin of the arch of the aorta just lateral to the ligamentum arteriosum. The left recurrent laryngeal nerve passes inferior to the arch of the aorta before ascending on its medial surface. Entering a groove between the trachea and esophagus, the left recurrent laryngeal nerve continues superiorly to enter the neck and terminate in the larynx (Fig. 3.96). The phrenic nerves arise in the cervical region mainly from the fourth, but also from the third and fifth, cervical spinal cord segments.


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Anatomy_Gray. The phrenic nerves arise in the cervical region mainly from the fourth, but also from the third and fifth, cervical spinal cord segments. The phrenic nerves descend through the thorax to supply motor and sensory innervation to the diaphragm and its associated membranes. As they pass through the thorax, they provide innervation through somatic afferent fibers to the mediastinal pleura, fibrous pericardium, and parietal layer of serous pericardium. The right phrenic nerve enters the superior mediastinum lateral to the right vagus nerve and lateral and slightly posterior to the beginning of the right brachiocephalic vein (see Fig. 3.94). It continues inferiorly along the right side of this vein and the right side of the superior vena cava.


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Anatomy_Gray. On entering the middle mediastinum, the right phrenic nerve descends along the right side of the pericardial sac, within the fibrous pericardium, anterior to the root of the right lung. The pericardiacophrenic vessels accompany it through most of its course in the thorax (see Fig. 3.60). It leaves the thorax by passing through the diaphragm with the inferior vena cava. The left phrenic nerve enters the superior mediastinum in a position similar to the path taken by the right phrenic nerve. It lies lateral to the left vagus nerve and lateral and slightly posterior to the beginning of the left brachiocephalic vein (see Fig. 3.89), and continues to descend across the left lateral surface of the arch of the aorta, passing superficially to the left vagus nerve and the left superior intercostal vein.


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Anatomy_Gray. On entering the middle mediastinum, the left phrenic nerve follows the left side of the pericardial sac, within the fibrous pericardium, anterior to the root of the left lung, and is accompanied by the pericardiacophrenic vessels (see Fig. 3.60). It leaves the thorax by piercing the diaphragm near the apex of the heart. Thoracic duct in the superior mediastinum The thoracic duct, which is the major lymphatic vessel in the body, passes through the posterior portion of the superior mediastinum (see Figs. 3.87 and 3.92). It: enters the superior mediastinum inferiorly, slightly to the left of the midline, having moved to this position just before leaving the posterior mediastinum opposite vertebral level TIV/V; and continues through the superior mediastinum, posterior to the arch of the aorta, and the initial portion of the left subclavian artery, between the esophagus and the left mediastinal part of the parietal pleura.


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Anatomy_Gray. The posterior mediastinum is posterior to the pericardial sac and diaphragm and anterior to the bodies of the mid and lower thoracic vertebrae (see Fig. 3.57). Its superior boundary is a transverse plane passing from the sternal angle to the intervertebral disc between vertebrae TIV and TV. Its inferior boundary is the diaphragm. Laterally, it is bordered by the mediastinal part of parietal pleura on either side. Superiorly, it is continuous with the superior mediastinum. Major structures in the posterior mediastinum include the: esophagus and its associated nerve plexus, thoracic aorta and its branches, azygos system of veins, thoracic duct and associated lymph nodes, sympathetic trunks, and thoracic splanchnic nerves. The esophagus is a muscular tube passing between the pharynx in the neck and the stomach in the abdomen. It begins at the inferior border of the cricoid cartilage, opposite vertebra CVI, and ends at the cardiac opening of the stomach, opposite vertebra TXI.


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Anatomy_Gray. The esophagus descends on the anterior aspect of the bodies of the vertebrae, generally in a midline position as it moves through the thorax (Fig. 3.97). As it approaches the diaphragm, it moves anteriorly and to the left, crossing from the right side of the thoracic aorta to eventually assume a position anterior to it. It then passes through the esophageal hiatus, an opening in the muscular part of the diaphragm, at vertebral level TX. The esophagus has a slight anterior-to-posterior curvature that parallels the thoracic portion of the vertebral column, and is secured superiorly by its attachment to the pharynx and inferiorly by its attachment to the diaphragm. Relationships to important structures in the posterior mediastinum In the posterior mediastinum, the esophagus is related to a number of important structures. The right side is covered by the mediastinal part of the parietal pleura.


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Anatomy_Gray. In the posterior mediastinum, the esophagus is related to a number of important structures. The right side is covered by the mediastinal part of the parietal pleura. Posterior to the esophagus, the thoracic duct is on the right side inferiorly, but crosses to the left more superiorly. Also on the left side of the esophagus is the thoracic aorta. Anterior to the esophagus, below the level of the tracheal bifurcation, are the right pulmonary artery and the left main bronchus. The esophagus then passes immediately posteriorly to the left atrium, separated from it only by pericardium. Inferior to the left atrium, the esophagus is related to the diaphragm. Structures other than the thoracic duct posterior to the esophagus include portions of the hemiazygos veins, the right posterior intercostal vessels, and, near the diaphragm, the thoracic aorta.


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Anatomy_Gray. Structures other than the thoracic duct posterior to the esophagus include portions of the hemiazygos veins, the right posterior intercostal vessels, and, near the diaphragm, the thoracic aorta. The esophagus is a flexible, muscular tube that can be compressed or narrowed by surrounding structures at four locations (Fig. 3.98): the junction of the esophagus with the pharynx in the neck; in the superior mediastinum where the esophagus is crossed by the arch of the aorta; in the posterior mediastinum where the esophagus is compressed by the left main bronchus; in the posterior mediastinum at the esophageal hiatus in the diaphragm.


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Anatomy_Gray. These constrictions have important clinical consequences. For example, a swallowed object is most likely to lodge at a constricted area. An ingested corrosive substance would move more slowly through a narrowed region, causing more damage at this site than elsewhere along the esophagus. Also, constrictions present problems during the passage of medical instruments. The arterial supply and venous drainage of the esophagus in the posterior mediastinum involve many vessels. Esophageal arteries arise from the thoracic aorta, bronchial arteries, and ascending branches of the left gastric artery in the abdomen. Venous drainage involves small vessels returning to the azygos vein, hemiazygos vein, and esophageal branches to the left gastric vein in the abdomen. Lymphatic drainage of the esophagus in the posterior mediastinum returns to posterior mediastinal and left gastric nodes.


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Anatomy_Gray. Lymphatic drainage of the esophagus in the posterior mediastinum returns to posterior mediastinal and left gastric nodes. Innervation of the esophagus, in general, is complex. Esophageal branches arise from the vagus nerves and sympathetic trunks. Striated muscle fibers in the superior portion of the esophagus originate from the branchial arches and are innervated by branchial efferents from the vagus nerves. Smooth muscle fibers are innervated by cranial components of the parasympathetic part of the autonomic division of the peripheral nervous system, visceral efferents from the vagus nerves. These are preganglionic fibers that synapse in the myenteric and submucosal plexuses of the enteric nervous system in the esophageal wall. Sensory innervation of the esophagus involves visceral afferent fibers originating in the vagus nerves, sympathetic trunks, and splanchnic nerves.


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Anatomy_Gray. Sensory innervation of the esophagus involves visceral afferent fibers originating in the vagus nerves, sympathetic trunks, and splanchnic nerves. The visceral afferents from the vagus nerves are involved in relaying information back to the central nervous system about normal physiological processes and reflex activities. They are not involved in the relay of pain recognition. The visceral afferents that pass through the sympathetic trunks and the splanchnic nerves are the primary participants in detection of esophageal pain and transmission of this information to various levels of the central nervous system.


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Anatomy_Gray. After passing posteriorly to the root of the lungs, the right and left vagus nerves approach the esophagus. As they reach the esophagus, each nerve divides into several branches that spread over this structure, forming the esophageal plexus (Fig. 3.99). There is some mixing of fibers from the two vagus nerves as the plexus continues inferiorly on the esophagus toward the diaphragm. Just above the diaphragm, fibers of the plexus converge to form two trunks: the anterior vagal trunk on the anterior surface of the esophagus, mainly from fibers originally in the left vagus nerve; the posterior vagal trunk on the posterior surface of the esophagus, mainly from fibers originally in the right vagus nerve. The vagal trunks continue on the surface of the esophagus as it passes through the diaphragm into the abdomen.


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Anatomy_Gray. The vagal trunks continue on the surface of the esophagus as it passes through the diaphragm into the abdomen. The thoracic portion of the descending aorta (thoracic aorta) begins at the lower edge of vertebra TIV, where it is continuous with the arch of the aorta. It ends anterior to the lower edge of vertebra TXII, where it passes through the aortic hiatus posterior to the diaphragm. Situated to the left of the vertebral column superiorly, it approaches the midline inferiorly, lying directly anterior to the lower thoracic vertebral bodies (Fig. 3.101). Throughout its course, it gives off a number of branches, which are summarized in Table 3.3. Azygos system of veins


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Anatomy_Gray. Azygos system of veins The azygos system of veins consists of a series of longitudinal vessels on each side of the body that drain blood from the body wall and move it superiorly to empty into the superior vena cava. Blood from some of the thoracic viscera may also enter the system, and there are anastomotic connections with abdominal veins. The longitudinal vessels may or may not be continuous and are connected to each other from side to side at various points throughout their course (Fig. 3.102). The azygos system of veins serves as an important anastomotic pathway capable of returning venous blood from the lower part of the body to the heart if the inferior vena cava is blocked. The major veins in the system are: the azygos vein, on the right; and the hemiazygos vein and the accessory hemiazygos vein, on the left. There is significant variation in the origin, course, tributaries, anastomoses, and termination of these vessels.


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Anatomy_Gray. There is significant variation in the origin, course, tributaries, anastomoses, and termination of these vessels. The azygos vein arises opposite vertebra LI or LII at the junction between the right ascending lumbar vein and the right subcostal vein (Fig. 3.102). It may also arise as a direct branch of the inferior vena cava, which is joined by a common trunk from the junction of the right ascending lumbar vein and the right subcostal vein. The azygos vein enters the thorax through the aortic hiatus of the diaphragm, or it enters through or posterior to the right crus of the diaphragm. It ascends through the posterior mediastinum, usually to the right of the thoracic duct. At approximately vertebral level TIV, it arches anteriorly, over the root of the right lung, to join the superior vena cava before the superior vena cava enters the pericardial sac.


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Anatomy_Gray. Tributaries of the azygos vein include: the right superior intercostal vein (a single vessel formed by the junction of the second, third, and fourth intercostal veins), fifth to eleventh right posterior intercostal veins, the hemiazygos vein, the accessory hemiazygos vein, esophageal veins, mediastinal veins, pericardial veins, and right bronchial veins. The hemiazygos vein (inferior hemiazygos vein) usually arises at the junction between the left ascending lumbar vein and the left subcostal vein (Fig. 3.102). It may also arise from either of these veins alone and often has a connection to the left renal vein. The hemiazygos vein usually enters the thorax through the left crus of the diaphragm, but may enter through the aortic hiatus. It ascends through the posterior mediastinum, on the left side, to approximately vertebral level TIX. At this point, it crosses the vertebral column, posterior to the thoracic aorta, esophagus, and thoracic duct, to enter the azygos vein.


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Anatomy_Gray. Tributaries joining the hemiazygos vein include: the lowest four or five left posterior intercostal veins, esophageal veins, and mediastinal veins. The accessory hemiazygos vein (superior hemiazygos vein) descends on the left side from the superior portion of the posterior mediastinum to approximately vertebral level TVIII (Fig. 3.102). At this point, it crosses the vertebral column to join the azygos vein, or ends in the hemiazygos vein, or has a connection to both veins. Usually, it also has a connection superiorly to the left superior intercostal vein. Vessels that drain into the accessory hemiazygos vein include: the fourth to eighth left posterior intercostal veins, and sometimes, the left bronchial veins. Thoracic duct in the posterior mediastinum


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Anatomy_Gray. Thoracic duct in the posterior mediastinum The thoracic duct is the principal channel through which lymph from most of the body is returned to the venous system. It begins as a confluence of lymph trunks in the abdomen, sometimes forming a saccular dilation referred to as the cisterna chyli (chyle cistern), which drains the abdominal viscera and walls, pelvis, perineum, and lower limbs. The thoracic duct extends from vertebra LII to the root of the neck. Entering the thorax, posterior to the aorta, through the aortic hiatus of the diaphragm, the thoracic duct ascends through the posterior mediastinum to the right of midline between the thoracic aorta on the left and the azygos vein on the right (Fig. 3.103). It lies posterior to the diaphragm and the esophagus and anterior to the bodies of the vertebrae. At vertebral level TV, the thoracic duct moves to the left of midline and enters the superior mediastinum. It continues through the superior mediastinum and into the neck.


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Anatomy_Gray. At vertebral level TV, the thoracic duct moves to the left of midline and enters the superior mediastinum. It continues through the superior mediastinum and into the neck. After being joined, in most cases, by the left jugular trunk, which drains the left side of the head and neck, and the left subclavian trunk, which drains the left upper limb, the thoracic duct empties into the junction of the left subclavian and left internal jugular veins. The thoracic duct usually receives the contents from: the confluence of lymph trunks in the abdomen, descending thoracic lymph trunks draining the lower six or seven intercostal spaces on both sides, upper intercostal lymph trunks draining the upper left five or six intercostal spaces, ducts from posterior mediastinal nodes, and ducts from posterior diaphragmatic nodes.


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Anatomy_Gray. The sympathetic trunks are an important component of the sympathetic part of the autonomic division of the peripheral nervous system and are usually considered a component of the posterior mediastinum as they pass through the thorax. This portion of the sympathetic trunks consists of two parallel cords punctuated by 11 or 12 ganglia (Fig. 3.104). The ganglia are connected to adjacent thoracic spinal nerves by white and gray rami communicantes and are numbered according to the thoracic spinal nerve with which they are associated. In the superior portion of the posterior mediastinum, the trunks are anterior to the neck of the ribs. Inferiorly, they become more medial in position until they lie on the lateral aspect of the vertebral bodies. The sympathetic trunks leave the thorax by passing posterior to the diaphragm under the medial arcuate ligament or through the crura of the diaphragm. Throughout their course the trunks are covered by parietal pleura. Branches from the ganglia


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Anatomy_Gray. Branches from the ganglia Two types of medial branches are given off by the ganglia: The first type includes branches from the upper five ganglia. The second type includes branches from the lower seven ganglia. The first type, which includes branches from the upper five ganglia, consists mainly of postganglionic sympathetic fibers, which supply the various thoracic viscera. These branches are relatively small, and also contain visceral afferent fibers. The second type, which includes branches from the lower seven ganglia, consists mainly of preganglionic sympathetic fibers, which supply the various abdominal and pelvic viscera. These branches are large, also carry visceral afferent fibers, and form the three thoracic splanchnic nerves referred to as the greater, lesser, and least splanchnic nerves (Fig. 3.104).


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Anatomy_Gray. The greater splanchnic nerve on each side usually arises from the fifth to ninth or tenth thoracic ganglia. It descends across the vertebral bodies moving in a medial direction, passes into the abdomen through the crus of the diaphragm, and ends in the celiac ganglion. The lesser splanchnic nerve usually arises from the ninth and tenth, or tenth and eleventh thoracic ganglia. It descends across the vertebral bodies moving in a medial direction, and passes into the abdomen through the crus of the diaphragm to end in the aorticorenal ganglion. The least splanchnic nerve (lowest splanchnic nerve) usually arises from the twelfth thoracic ganglion. It descends and passes into the abdomen through the crus of the diaphragm to end in the renal plexus.


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Anatomy_Gray. The ability to visualize how anatomical structures in the thorax are related to surface features is fundamental to a physical examination. Landmarks on the body’s surface can be used to locate deep structures and to assess function by auscultation and percussion. How to count ribs Knowing how to count ribs is important because different ribs provide palpable landmarks for the positions of deeper structures. To determine the location of specific ribs, palpate the jugular notch at the superior extent of the manubrium of the sternum. Move down the sternum until a ridge is felt. This ridge is the sternal angle, which identifies the articulation between the manubrium of the sternum and the body of the sternum. The costal cartilage of rib II articulates with the sternum at this location. Identify rib II. Then continue counting the ribs, moving in a downward and lateral direction (Fig. 3.105). Surface anatomy of the breast in women


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Anatomy_Gray. Surface anatomy of the breast in women Although breasts vary in size, they are normally positioned on the thoracic wall between ribs II and VI and overlie the pectoralis major muscles. Each mammary gland extends superolaterally around the lower margin of the pectoralis major muscle and enters the axilla (Fig. 3.106). This portion of the gland is the axillary tail or axillary process. The positions of the nipple and areola vary relative to the chest wall depending on breast size. Visualizing structures at the TIV/V The TIV/V vertebral level is a transverse plane that passes through the sternal angle on the anterior chest wall and the intervertebral disc between TIV and TV vertebrae posteriorly. This plane can easily be located, because the joint between the manubrium of the sternum and the body of the sternum forms a distinct bony protuberance that can be palpated. At the TIV/V level (Fig. 3.107): The costal cartilage of rib II articulates with the sternum.


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Anatomy_Gray. The costal cartilage of rib II articulates with the sternum. The superior mediastinum is separated from the inferior mediastinum. The ascending aorta ends and the arch of the aorta begins. The arch of the aorta ends and the thoracic aorta begins. The trachea bifurcates. Visualizing structures in the superior mediastinum A number of structures in the superior mediastinum in adults can be visualized based on their positions relative to skeletal landmarks that can be palpated through the skin (Fig. 3.108). On each side, the internal jugular and subclavian veins join to form the brachiocephalic veins behind the sternal ends of the clavicles near the sternoclavicular joints. The left brachiocephalic vein crosses from left to right behind the manubrium of the sternum. The brachiocephalic veins unite to form the superior vena cava behind the lower border of the costal cartilage of the right first rib.


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Anatomy_Gray. The brachiocephalic veins unite to form the superior vena cava behind the lower border of the costal cartilage of the right first rib. The arch of the aorta begins and ends at the transverse plane between the sternal angle anteriorly and vertebral level TIV/V posteriorly. The arch may reach as high as the midlevel of the manubrium of the sternum. Visualizing the margins of the heart Surface landmarks can be palpated to visualize the outline of the heart (Fig. 3.109). The upper limit of the heart reaches as high as the third costal cartilage on the right side of the sternum and the second intercostal space on the left side of the sternum. The right margin of the heart extends from the right third costal cartilage to near the right sixth costal cartilage. The left margin of the heart descends laterally from the second intercostal space to the apex located near the midclavicular line in the fifth intercostal space.