Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 37

The cornea is the transparent, anterior portion of the fibrous tunic of the globe. Like the lens, the cornea is normally clear, and transmits and refracts light (40–42 diopters in dogs). The avascular cornea relies on both the aqueous humor and tear film for nourishment and on the eyelids and NM for protection from the external environment. The cornea is elliptical in shape, with a horizontal diameter greater than the vertical (Table 1.8). In the dog and the cat, the difference between these diameters 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).