Mar 1, 2019
Every student of anatomy has heard it a thousand times: "Structure Follows Function." We need only to study the structure of a particular organ to glean precious insight into its func-tion. When looked at closely, the structure of the eye is nothing less than astounding. Be-fore we can discuss the health or disease of the eye, we must consider its structure.
Let’s start with the globe itself: the "eyeball" is made of a tough connective tissue that re-tains its shape and size even when subjected to constant changes in pressure. This material makes up the white sclera of the globe. With every pulse, blood entering the eye causes changes in the pressure and internal volume of the eye, and yet, we do not perceive changes in our visual image. The eye is like a camera built out of a balloon, yet it still takes fantastic pictures.
The globe is divided into the anterior and vitreous chambers. The anterior chamber occu-pies the forward aspect of the globe and contains the cornea, a clear window through which light enters the eye. The cornea is attached to the sclera at the limbus, which is cru-cially important because this is the filter where fluid leaves the inside of the globe. Behind the cornea is the iris, the colorful muscle that enlarges or constricts the pupil, which is the hole through which light passes on its way to the lens and vitreous chamber. The lens is supported by suspensory ligaments which attach to the muscular ciliary body. It is the cili-ary body that stretches the lens and allows us to focus on objects in our near field of vision. The ciliary body is also the source of the never-ending river of fluid produced inside the eye which maintains pressure and inflation of the globe.
Whew! That’s a lot of anatomy, and we haven’t even made it half way back to the retina! But knowing just these structures, we can now begin to talk about diseases of the eye itself.
The cornea is right up front, so it is easily wounded. Corneal ulcers are quite common and occur when objects strike the corneal surface. Foreign bodies, particularly foxtails (self-planting grass seeds), have an amazing ability to work themselves under the eyelid, where they scrape away the surface of the cornea. Most corneal ulcers heal quickly, but if the cor-nea becomes infected, it may erode to the point that the eye is perforated and deflates. When this happens, the structures of the anterior chamber often stick together, disrupting the func-tion of the optic camera. This is the reason that corneal injuries are treated as emergencies. Sure, they usually heal quickly, but when a corneal problem goes wrong, it goes very, very wrong and the eye can easily be damaged beyond repair.
Another common disease of the cornea is keratoconjunctivitis sicca, (KCS aka "dry eye") which is actually a disease of tear glands resulting in loss of tear production. Without an adequate tear film, the cornea is ill-nourished and becomes irritated or infected, and often ulcerates. If KCS is left uncontrolled, the dryness causes the cornea to thicken and become pigmented, resulting in blindness.
Problems involving the iris are relatively uncommon. Iris dysfunction usually results from a problem with the nerves that control iris muscle rather than the iris itself. Iris atrophy, the age related thinning of the iris may result in holes developing in the organ, but typically causes little trouble. Pigmented tumors (melanoma) of the iris are fairly common, but most have benign behavior and require little or no treatment.
The lens is another matter. Diseases of the lens are many and common. Age itself leads to drying of the lens tissue, which decreases lens flexibility and leads to far-sightedness. When the drying process is more severe it creates the noticeable fogging of the lens (nucle-ar sclerosis) seen in many of our senior dogs. High blood glucose levels in diabetes allow excess sugar to enter the lens, which attracts water and fogs the lens through a completely different mechanism. In their most extreme form, these lens changes result in the develop-ment of cataracts, where lens clarity and some visual ability are lost.
Two entire columns and we haven’t even made our way into the vitreous chamber! More on the eye next month!
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