One of the functions of the eyeball is to keep its shape. Or to put it another way, to keep distended to the right size.
The eye is an optical instrument which, like all optical instruments, depends for its usefulness on focusing rays of light through a lens upon a sensitive surface. In order to do this the chamber of any optical instrument must have a certain depth. The eyeball is the chamber of the eye; it contains a clear fluid and the tension of this fluid is kept constant.
If the fluid in the eye diminishes the eye collapses like an empty balloon. The vision is gone. In like manner, if the fluid increases the eyeball is distended and the vision is impaired.
The amount of fluid in the eye is kept at a definite level and at a definite tension by a very delicate mechanism. The fluid in the anterior part of the eye, between the lens and the cornea, is derived not from blood, but from lymph, and the tension of this fluid is kept the same by the action of lymph vessels in the margin of the pupil muscle, or iris.
When this mechanism is disturbed the fluid in the eyeball is not drained away sufficiently fast. The result is an increase in pressure. The disease is known as glaucoma.
Glaucoma is a fairly frequent eye disease of middle age. Its presence should always be considered when a person over 40 begins to have dimness of vision and pain in the eye, or headache.
Indeed, it should always be thought of in any person over 40 who has to keep changing glasses too frequently, and who uses glasses stronger than the age accounts for.
There are many different forms of glaucoma, but they all depend upon this increase in pressure, due to the accumulation of fluid in the eye. Blood and lymph keep pumping into the eye every second, and if there were no way to get rid of it the eyeball would burst. This actually happens in some eyes when the outer coat of the eyeball has become weakened. Acute glaucoma comes in attacks in which the eye is congested and shows external signs of disturbance. The attack passes off in a few days, only to recur. In simple glaucoma the process is more gradual.
In either case treatment should be instituted immediately in order to preserve vision. About 12 per cent of all blindness is caused by glaucoma. Much of it can be prevented by modern treatment.
“Perhaps this account,” a Doctor Jackson says, in a health leaflet on glaucoma, “resembles the descriptions of disease given in patent medicine almanacs, designed to make every one think he has the disease and needs the medicine. But the only way to prevent blindness and suffering from glaucoma is to have a great many people know that there is such a disease and that generally it can be checked.”