Essentials of Veterinary Ophthalmology. Kirk N. Gelatt

near the optic disc (peripapillarily). This pattern is seen in certain ungulates, such as horses, elephants, and rhinoceroses, and in some marsupials such as kangaroos. The anangiotic pattern is characterized by an absence of any vasculature within the neurosensory retina, and it occurs in sugar gliders, guinea pigs, chinchillas, and nonmammalian species such as birds (Figure 1.56a and b). In birds, a structure called pectin (pectin oculi) lies vitread to the optic nerve head. It is pigmented and pleated structure, and contains a rich plexus of blood vessels. In general, the retinal arterial supply in domestic animals comes from the short posterior ciliary arteries, which are termed cilioretinal arteries, rather than via a central retinal artery origin as in higher primates, rats, and mice.

Figure 1.56 (a) The avian pecten, as seen here in the chicken, consists of a pleated vascular plexus that lies vitread atop the optic nerve head (ON). (Original magnification, 50×.) (b) Close‐up of the base of the pecten as it internally lines the nerve fibers (NF) that form the optic nerve head. BV, blood vessels of the pecten. (Original magnification, 250×.)

Optic Nerve

Retinal ganglion cell axons leave the nerve fiber layer and form the optic disc. From this area, they pass through the choroid and sclera and into the orbit as the optic nerve. In addition to ganglion cell axons, the optic nerve is composed of glial cells and septae, which arise from the pia mater. The visual axons synapse in the lateral geniculate nucleus, whereas the pupillomotor fibers synapse in the nucleus of CN III. The optic nerve extends from the globe to the optic chiasm, and it consists of four regions: intraocular, intraorbital, intracanalicular, and intracranial (Figure 1.57). Because of similar anatomical properties, the optic nerve is considered to be more of a nerve fiber tract of the brain than a peripheral nerve. The intraocular optic nerve consists of retinal, choroidal, and scleral portions.

      The terms optic disc, papilla, and optic nerve head are interchangeable and include the retinal and choroidal portions of the optic nerve. Optic papilla refers to an elevation of the nerve head, and its presence and development vary among and within species. Within the optic papilla is a central depression called the physiologic cup. The cup is lined by a plaque of astrocytes known as the central supporting tissue meniscus of Kuhnt. An exaggeration of this tissue is Bergmeister's papilla, which is the remnant of the hyaloid artery on the disc's surface.

      The number of optic nerve fibers, and their density and size vary considerably among species. Animals with poorly developed eyes, such as mole rats, contain approximately 900–1800 nerve fibers, whereas those with highly developed eyes, such as various primates, have 100–150 times that number. Interestingly, the size of the eye often does not correlate with the total number of nerve fibers within the optic nerve.

Figure 1.57 The optic nerve head and bulbar optic nerve of a dog. Arrows indicate lamina cribrosa; note the number of astrocytes anterior to it. C, choroid; CMK, central meniscus of Kuhnt (accumulation of astrocytes in physiological cup); CRV, central retinal vein; RV, retinal veins; S, sclera; PS, pial septa. (Original magnification, 720×.)

Vasculature of the Eye and Orbit

      Among domestic animals, the main supply of blood to the eye and orbit is via the internal maxillary artery (a branch of the external carotid artery), which after passing through the alar canal branches to give rise to the external ophthalmic artery. By comparison, in primates, the entire microcirculation of the eye and most of the orbital circulation are supplied via the internal carotid artery, which gives rise to the internal ophthalmic artery.

      Domestic species possess both internal and external ophthalmic arteries, but the external ophthalmic artery provides most of the circulation to the eye. Both the long and short posterior ciliary arteries as well as the lacrimal, muscular, and supraorbital arteries derive from the external ophthalmic artery. The internal ophthalmic artery, which is relatively small, provides the blood supply for the optic nerve and anastomoses with the external ophthalmic artery or one of its branches; this anastomosis is especially prominent in the dog.

2 Ophthalmic Physiology and Vision

       Revised from the 6th edition of Veterinary Ophthalmology, Chapter 3: Physiology of the Eye, by Diane V.H. Hendrix, Sara M. Thomasy, and Glenwood G. Gum; Chapter 4: Optics and Physiology of Vision, by Ron Ofri and Björn Ekesten; and Chapter 5: Vision, by Björn Ekesten and Ron Ofri

Section I: Physiology of the Eye

      Functional knowledge of ocular physiology in animal species provides a critical foundation for clinicians practicing comparative and veterinary ophthalmology. This chapter presents the physiology of the eye, especially regarding the adnexa, anterior segment, ocular circulation, aqueous humor (AH) dynamics, lens, and vitreous; optics; and vision.

      The rate at which both relatively simple and complex ocular physiological mechanisms are being studied is incredible. This chapter presents the essential physiological phenomena of the eye, optics, and vision required by the clinical veterinary ophthalmologist.

Anterior Eye Structures

      Eyelids

      The eyelids of domestic animals protect the eye, particularly the cornea. All domestic animal species have a superior (upper) and an inferior (lower) eyelid; most have a nictitating membrane (NM, third eyelid). The eyelids contain the meibomian glands; these are large sebaceous glands that secrete the outer, oily layer of the precorneal tear film (PTF). The conjunctiva lines the inside of the eyelids and reflects onto the globe contains goblet cells that contribute the mucin to the PTF; accessory lacrimal glands are also present in some species. The normal blinking of the eyelids maintains the physiological thickness of the preocular tear film, aids movement of the tears both to and within the nasolacrimal system, and helps eliminate small particles from the corneal and conjunctival surfaces. Reflex closure of the eyelids protects the anterior segment from external trauma.

      The eyelids determine the shape and width of the palpebral fissure, along with the associated medial and lateral canthal ligamentous and muscle attachments. For example, a wide, round palpebral fissure is normal among brachycephalic breeds, and a narrow, almond‐shaped palpebral fissure is normal among dolichocephalic breeds. The shape of the palpebral fissure also depends on the relationship of the globe to the orbit. A small globe in a deep orbit allows a narrow palpebral fissure; the opposite occurs with a large globe in a shallow orbit. The NM aids in protection of the conjunctiva and cornea by moving, either passively or actively, over the cornea when the globe is retracted.

Eyelid closure is mediated by the efferent fibers of the facial nerve (CN VII) and their effects on the orbicularis oculi muscles. The oculomotor nerve (CN III) innervates the levator palpebral superioris, which is responsible for opening the upper eyelids. Eyelid closure is the end result of two eyelid reflexes, the corneal and palpebral reflexes, and the menace response (Table 2.1). The corneal and palpebral reflexes are primitive reflexes with a purely subcortical course. Both are elicited by touch, with the afferent pathway being the ophthalmic branch of the trigeminal nerve (corneal) or the ophthalmic and maxillary branches of the trigeminal nerve (palpebral). The efferent pathway