Essentials of Veterinary Ophthalmology. Kirk N. Gelatt
until it reaches an opening at the floor of the nostril approximately 1 cm from the end of the nares. Approximately 40% of dogs have an accessory opening in the canal as it passes by the root of the upper canine tooth.
The lacrimal gland is a diamond‐shaped structure in the dorsolateral aspect of the orbit underneath the orbital ligament. The mean length, width, thickness, and weight of the relatively flat lacrimal gland in three different breeds of dogs were ~17 ± 0.7 mm, ~13 ± 0.4 mm, ~3 ± 0.1 mm, and ~316 ± 21 mg, respectively. Fifteen to twenty small ductules drain into the superior conjunctival fornix. Histologically, the gland is a tubuloalveolar type. The innervation to the lacrimal gland is not fully understood, but the lacrimal branch of cranial nerve V, and sympathetic and parasympathetic nerves are all involved in its function. Clinically, certain cholinergic drugs (e.g., pilocarpine) stimulate tear secretion, whereas other drugs (i.e., anticholinergics) decrease tear secretion.
Figure 1.16 The nasolacrimal system: lacrimal puncta, canaliculi, lacrimal sac, nasolacrimal duct, lacrimal gland, and lacrimal ducts.
Figure 1.17 Diagram of the three tunics that comprise the mammalian globe. Outermost fibrous tunic (light and dark purple), consisting of the cornea and sclera; the middle tunic called the uvea (light orange), consisting of the iris, ciliary body, and choroid; and the nervous tunic (dark orange) consisting of the retina and optic nerve.
Globe
Components
The globe is composed of three basic layers or coats (Figure 1.17). The outer layer is the fibrous tunic, which is further divided into the cornea and sclera. The fibrous tunic provides shape to the eye. In addition, the anterior portion of the fibrous tunic (i.e., the cornea) is transparent, thus enabling light to pass through, and is shaped in a manner that makes it a powerful lens that refracts light rays centrally, toward the visual axis of the eye.
The middle layer is the vascular tunic, called the uvea (meaning “grape”). The uvea is further divided into the iris, ciliary body, and choroid, and is heavily pigmented and vascularized. It functions to restrict the amount of light entering the eye and to provide nourishment and remove waste products.
The innermost layer is the nervous tunic, which consists of the retina and optic nerve. The three tunics embrace the large, inner, transparent media of the eye: the aqueous humor, lens, and vitreous humor, which collectively function to transmit and refract light to the retina and provide an internal pressure that keeps the globe firmly distended.
Size, Shape, and Topography
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 |
Cornea
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