Anatomic and physiologic considerations are the basis for proper diagnostic techniques. We will discuss basic diagnostic procedures
and relative pharmacological consideration to enhance the ophthalmic examination.
Normal anatomy and physiology
The eye is a sensitive neurological tissue and is considered the anterior extension of the brain. It functions to translate
light waves into a nerve impulse, which can then be interpreted by the brain as sight. The eye is extremely delicate and sensitive,
and has special anatomic and physiologic adaptations. A basic understanding of ocular anatomy and physiology are necessary
to appreciate abnormalities and understand their significance. The eyes and their associated structures are bilateral and
symmetrical in most species.
Eyelids protect, lubricate and meter out excessive light. The shape of the palpebral fissure is species and breed dependent.
A wide variation among the breeds results in a particular facial expression. Two main sets of muscles control eyelid position.
The orbicularis muscles, innervated by the facial nerve, close the eyelid. The levator palpebral muscle innervated by the
oculomotor nerve raises the upper eyelid. The margins of the eyelids contain specialized cilia known as eyelashes, and openings
to the sebaceous meibomian glands provide oily secretions. The glands of Zeis and Moll are sebaceous and sweat glands that
are associated with the follicles of the eyelash.
The conjunctiva is the mucous membrane lining inside of the eyelids (palpebral conjunctiva) and is continuous onto the anterior
sclera (bulbar conjunctiva). The palpebral conjunctiva is thicker and pink-red in color, whereas the bulbar conjunctiva is
very thin, colorless and nearly transparent. Both contain blood vessels which when irritated will become congested and red.
Jaundice contrition will give the conjunctiva a yellowish color. Anemia will give it a blanched, pale appearance. The third
eyelid (nictitating membrane) is a specialized conjunctival structure of non-primate animals. It serves to protect the cornea,
secrete part of the tear film and fight infection via lymph follicles. This is supported by a T-shaped piece of cartilage
and at the base of this cartilage is the gland of the third eyelid, which produces a significant quantity of tears. The excursion
of the nictitating membrane, or third eyelid, is passive in dogs and due to the retraction of the globe.
Lacrimation is the production of tears. Tears are a composite of secretions of several glands. The two main glands in small
animals are their orbital lacrimal gland located in the dorso-temporal orbit and the gland of the third eyelid. Tear secretion
provides a protective function (antibacterial activity) and a source of nutrition and wastes product removal for the cornea.
The tear film is comprosed of the trilaminar layer: the out oily layer (from the meibomian glands), the middle aqueous layer
(from the lacrimal gland and gland of the third eyelid), and an inner mucin layer (conjunctival glands). A lack of watery,
aqueous tear secretion results in keratoconjuctivitis sicca, while a lack of mucin secretion results in abnormalities of the
tear film and focal areas of dryness. Chronic blepharitis may compromise meibomian gland secretions. This will decrease the
outer, oily layer, resulting in more rapid evaporation of the aqueous layer.
Tears exit the palpebral fissure by evaporation or through the nasolacrimal system. The openings of the apparatus are located
at the medial canthi and are comprosed of a small opening (punctum) in the upper and lower eyelids. The puncta, via canaliculi
(lacrimal ducts), join to forma a lacrimal sac. The nasolacrimal drainage duct exists from the lacrimal sac, passes through
the bones of the nose, and exits trough the nasal punctum; generally, a few millimeters from the external nares.
The eye proper is composed of three layers or tunics: (1) the outer fibrous tunic is composed of the corneo-scleral shell;
(2) the middle vascular tunic is comprosed of the choroids, Ciliary body, and iris; and (3) the inner nervous tunic, which
is of neural tissue origin and is comprosed of the neurosensory retina, retinal pigment epithelium of the Ciliary body, and
the posterior iris surfaces.