Essentials of Veterinary Ophthalmology. Kirk N. Gelatt

is small (<1–2 mm), thus making their corneas appear almost circular. In most ungulates, this difference is much more pronounced, allowing for a remarkable horizontal field of view that is further complemented by the lateral positioning of the orbits, and greater protection from predators.

Figure 1.18 (a) Lateral view of the equine globe. Note the marked flattening in the anteroposterior axis and the marked ventral exit of the optic nerve from the posterior pole. (b) Posterior view of a canine globe. LP, long posterior ciliary artery; ON, optic nerve.

Table 1.8 Width and height (mm) of the cornea measured in a straight line.

Animal	Width	Height	Ratio of height to width
Horse	34.0	26.5	1:1.28
	33.1	25.8
Cow	30.5	23.2	1:1.29
Sheep	22.4	15.4	1:1.45
Pig	17.7	14.7	1:1.20
Dog	16.3	15.25	1:1.07
Cat	17.0	16.0	1:1.07

      Corneal thickness varies between species, breeds, individuals, and location (i.e., central versus peripheral cornea). In most domestic animals, it is less than 1 mm thick. Corneal thickness is also influenced by age and time of day. Corneal thickness increases significantly with age in the dog, cat, and horse.

      The cornea is richly supplied with sensory nerves, particularly pain receptors, and this sensitivity provides protection to the cornea and helps maintain its transparency. The cornea is innervated by the long ciliary nerves, which are derived from the ophthalmic branch of the trigeminal nerve. The epithelial cell layers are richly innervated, and these nerve endings are unsheathed among the epithelia. Use of immunohistochemical localization of neuropeptides associated with the ciliary ganglion in the dog has revealed the presence of a well‐developed pattern of epithelial innervation consisting of numerous horizontally oriented leash formations at the level of the epithelial basal cells, but the stromal innervations, which exist superficially, consist of main bundles that repetitively branch in a dichotomous manner to create elaborate axonal arborizations. In general, the most superficial layers are primarily innervated with pain receptors, whereas more pressure receptors are found in the stroma. This explains why a superficial corneal injury is often more painful than a deeper wound.

The cornea comprises four (sometimes five) layers. From superficial to deep, the layers are the epithelium, Bowman's layer (in some species), stroma, Descemet's membrane, and endothelium (Figure 1.19). The transparency of the cornea results from lack of blood vessels, nonkeratinized surface epithelium maintained by a PTF, relative dehydration (deturgescence), and the size and organization of stromal collagen fibrils.

Figure 1.19 Histological view of the four layers in the equine cornea: anterior epithelium (AE), stroma (S), Descemet's membrane (DM), and endothelium (E). Inset: Basal cells (B), wing cells (W), and squamous cells (S).

      Corneal Epithelium

The corneal epithelium is a nonkeratinized, stratified squamous epithelium that covers the anterior corneal surface. The epithelium is approximately 25–40 μm thick in the domestic carnivore and two to four times thicker in the ungulate. In the dog, cat, and birds, the anterior epithelium consists of a single layer of basal cells that lie on a thin basement membrane (Figure 1.20a and b), two or three layers of polyhedral (i.e., wing) cells, and two or three layers of nonkeratinized squamous cells. In larger animals, the layers of polyhedral and squamous cells are more numerous. The cells are arranged to provide orderly replacement of the surface cells during desquamation.

There are several layers of outer flattened superficial squamous cells. The cells appear to be flat and polygonal with straight borders on scanning electron microscopy (SEM) (Figure 1.21). Both light and dark cell types can be identified. The light cells contain more microvillae and microplicae. These numerous projections scatter electrons and, as a result, produce a lighter appearance of the cell. The darker cells are older and are occasionally seen to be desquamating (see Figure 1.15b). Cells in the central cornea have more projections (i.e., microplicae and microvillae) than those in the periphery. It has been proposed that the fine microplicae and microvillae that considerably expand the cells' surface area enable movement of oxygen, potential nutrients, and various metabolic products across the exposed cell membranes of the outermost squamous epithelial cells. Also more likely, the microprojections of the squamous epithelial cells, which can be sometimes intricate in their patterns, allow mucin of the PTF to adhere firmly to the anterior epithelium, which aids in stabilizing the tear film on the corneal surface.

Beneath the epithelium is a basement membrane, which stains positively with PAS (see Figure 1.20). The basal cells are firmly attached to the basal lamina of the basement membrane (i.e., anterior limiting lamina) by hemidesmosomes, anchoring collagen fibrils, and the glycoprotein laminin. Ultrastructurally, the basement membrane consists of a 30–55 nm thick osmiophilic layer that is separated from the basal cell plasma membrane by a 25 nm wide, electron‐lucent zone (see Figure 1.15b). Hemidesmosomes attach the basal cells to the basement membrane, which in turn anchors the epithelium to the stroma. The arrangement of hemidesmosomes varies among different animals, being linear among mammals and amphibians, in rosettes among birds and reptiles, and punctate without arrangement, or completely absent, among fish. The epithelial cells have strong regenerative abilities (basal cell turnover time is approximately seven days), but after removal of the basal lamina, weeks to months may be necessary for it to completely reestablish.

Figure 1.20 Basement membrane (arrows) of the anterior epithelium