The outer coat of the eye consists of opaque sclera and transparent cornea. The cornea has five layers: 1) Precorneal tear film; 2) epithelium and its basement membrane; 3) stroma; 4) Descemet's membrane (basement membrane of the endothelium); 5) endothelium.2
Dog and cat corneas are relatively large compared to a man's, probably to assist in night vision. Animals with large corneas are typically nocturnal, as large corneas allow greater amounts of light to enter the pupil during reduced illumination. Most animal corneas are roughly elliptical in shape with the vertical diameter slightly less than the horizontal diameter. The normal dog cornea measures 12-16 mm vertically and 13-17 mm horizontally. Corneal measurements, both diameters and thickness, have not been established for different ages and different breeds in either dog or cat.1
The cornea, along with sclera, forms the fibrous tunic for the globe. The zone where the cornea gradually becomes opaque and changes to sclera is the limbus. The dog and cat corneas are divided into axial (central) and peripheral with the central area most important for vision. The central cornea is generally the thinnest and most often affected with ulcerations.
Diseases alter corneal transparency. The invasion of cornea with blood vessels; accumulation of fluids and swelling; infiltration with different types of leukocytes; migration of pigment cells from the limbus and conjunctiva; and deposition of fat, cholesterol and calcium products reduce the cornea's ability to transmit images.1
Congenital Anomalies Linked to Abnormal Cornea
Changes in cornea size and contour usually arise as a congenital anomaly. Abnormal corneal size, in the form of either a small or large cornea, in an otherwise normal globe, is rare. Microcornea is more frequently associated with microphthalmia and multiocular anomalies for which there is a breed predisposition in Old English Sheepdog, Saint Bernard, Australian Shepherd, Cocker Spaniel and Rough Collie.
Megalocornea (enlarged cornea) is usually associated with congenital glaucoma, a very uncommon condition in the dog.
- Small animal ophthalmic surgery: practical techniques for the veterinarian By Janice Peterson Gelatt
- Fundamentals of veterinary ophthalmology. Douglas H. Slatter