The lens is a clear crystalline object covered by a fine capsule. At the outer periphery, the capsule connects with the ciliary body also. As the ciliary body contracts or dilates, it can alter marginally the shape of the lens. This gives the lens its power of accommodation. In other words, it tends to keep objects in focus whether they are close or at a distance. This is an automatic function, and reflexively occurs without having to give it any thought. It is an ingenious device, one the camera-makers have been trying to emulate for many years.
There are two important spaces in front of the lens. A large one, termed the anterior (or front) chamber, exists between the cornea and the iris. It is filled clear fluid. Between the iris and the lens is a much smaller space called the posterior (or back) chamber. The two are continuous and connect through the pupil, the dark black part in the centre of the eye, through which light passes to the lens. An active circulation occurs in the chambers of the eye. The ciliary processes of the ciliary body secrete a fluid called aqueous humor. This flows through the lens, and from the posterior chamber enters the anterior chamber through the pupil. Once it has gained access to the anterior chamber, it tends to flow to the periphery to be drained off into tiny canals and enter the general body fluid circulation.
This circulation of fluid in the eye is extremely important, for if either excess fluid is formed or there is a block to the outflow system, pressure can build up it the eyeball. This can lead on to a serious condition called glaucoma, which can gradually and permanently destroy normal vision.
The rear two-thirds of the eyeball filled with a clear jellylike fluid, made up of water and certain chemicals, and this is referred to as the vitreous. The back part of the eye is the light sensitive retina. This is actually the expanded optic nerve that penetrates the eyeball at a point called the optic disc. This rapidly gives rise to an enormous sheet tissue, complete with light-sensitive organs, entering the eyeball in conjunction with the optic nerve are the central retinal vessels, the artery and vein. Immediately they gain access to the eyeball, they also divide into an intricate network of vessels. The artery conveys blood to the eye, and the vein brings the spent blood back to the circulation of the body.