Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 62

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×.)