Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 36

The eyes in domestic animals are quite variable in size, but their shapes are comparatively uniform, being spherical in most instances, in which the three axes of the globe (anteroposterior, horizontal or transverse, and vertical) are nearly identical in dimensions (Table 1.7). Some of the larger ungulates, including the cow and horse, possess globes that are relatively flattened in the anteroposterior axis. Two principal planes, the equatorial and meridional, are traditionally used in references to the three axes. The equatorial plane bisects the anterior and posterior poles,

and is perpendicular to the meridional plane. Any plane that runs parallel to the equatorial plane is called the frontal, coronal, radial, or transverse plane. The meridional plane moves along the anteroposterior axis of the eye, vertically dividing it into medial and lateral halves, even though meridional planes can be horizontal or oblique. Planes that run parallel to the meridional plane are described as sagittal planes.

The optic nerve in most domestic animals lies inferior and lateral to the posterior pole (Figure 1.18a and b). Surrounding the optic nerve are many ciliary nerves and short posterior ciliary arteries. In normal dogs, the mean number of short posterior ciliary arteries is 12 (~7 dorsally and ~5 ventrally). The posterior ciliary nerves pursue a long intrascleral course (up to 12 mm) at the 9‐ and 3‐o'clock positions before entering the suprachoroidal space to reach the iris, ciliary body, and limbus. In the dog, the long posterior ciliary arteries enter the sclera approximately 3–5 mm from the optic nerve in the horizontal meridian. In the cat, these arteries can enter the sclera immediately adjacent to the optic nerve. Recurrent vascular branches enter the choroid, but the main vessel trunk continues to be the major supply to the iris. A variable number of vortex veins (usually four) emerge from the sclera posterior to the equator; typically, two vortex veins are present dorsally and two ventrally.

Table 1.7 External globe dimensions (mm).

Animal	Meridional anteroposterior axis of the eye, A (mm)	Equatorial axis, V (mm)	Horizontal, T (mm)	Ratio of A/V/T	Ratio of V/T
Horse	43.68	47.63	48.45	1:1.09:1.10	1:1.10
Cow	35.34	40.82	41.90	1:1.15:1.18	1:1.02
Sheep	26.85	30.02	30.86	1:1.11:1.15	1:1.02
Pig	24.60	26.53	26.23	1:1.08:1.06	1:0.99
Dog	21.73	21.34	21.17	1:0.98:0.97	1:0.99
Cat	21.30	20.60	20.55	1:0.97:0.96	1:0.99