Photos courtesy of Long Beach Animal Hospital
The cornea is a tough translucent structure about one millimeter thick. It protects the front of the eye while simultaneously allowing light to pass through to the retina. Because of its required transparency, it cannot have a blood supply, so oxygen and nutrients to keep the cornea cells healthy and alive have to diffuse through the tear film on the outer surface. The inner surface of the cornea receives its nutrition from the aqueous humor (fluid on the inside of the eye) on the inner surface.
The transparent cornea is made of several layers. The outer epithelium is the clear part you see when you look at the eye. The stroma, the next layer down, is the part that has collagen fibers and holds the cornea together and still lets light pass through to the retina. Another and final layer is called Descemet’s membrane. It is the innermost layer of the cornea. This layer is only one cell thick. It prevents fluid in the eye from leaking out and is the last line of defense to keep the eye intact.
This owl cornea is clear enough to let light pass into the retina, yet it’s tough enough to protect the eye. It is an amazing piece of anatomy.
For a fun comparison to the owl, this is a human cornea.
Brachycephalic breeds—dogs with short heads and muzzles, like pugs, Pekingese and bulldogs—have a greater risk of eye injury. Other examples of these animals are shih-tzu, Boston terriers, Cavalier King Charles Spaniels, and Persian and exotic shorthair cats. Because of the shape of the shortened skull, the eye sockets are shallower than normal and the eyeball doesn’t sit as deeply in the head. The corneal surface is not as well protected and is subject to more trauma than usual.
Some brachycephalic breeds also have a tendency for inherited eye diseases. These diseases cause the eyelashes to rub against the cornea. The combination of these defects plus the shallow eye socket makes these animals more prone to corneal trauma.
Also at higher risk are cats in shelters or rescue situations, where large numbers are kept fairly close together and contagious respiratory diseases spread readily. These infections often lead to secondary bacterial conjunctivitis (infection of the membranes surrounding the eyes), increasing the risk of corneal trauma.
In next week’s column, we’ll discuss the treatment of corneal ulcers.