9. THE ORGAN OF VISION

The eye consists of the eyeball and the auxiliary apparatus surrounding it.

The eyeball is spherical in shape and is situated in the eye socket. The eyeball has three coats surrounding its inner nucleus: a fibrous outer coat, a vascular middle coat, and an inner reticular coat (the retina).

The fibrous coat forms an external sheath around the eyeball and plays a protective role. In the posterior, the largest of its parts, it forms an opaque tunic called the sclera, and in the anterior segment, a transparent cornea. Both areas of the fibrous coat are separated one from the other by a shallow circular sulcus.

The sclera consists of dense connective tissue, white in colour. Its anterior part is visible between the eyelids, and is commonly referred to as the "white of the eye".

The cornea is a continuation of the sclera. It is fitted by its edge into the anterior segment of the sclera.

The vascular coat of the eyeball is rich in vessels, soft, dark-coloured by the pigment contained in it. It lies immediately under the sclera and consists of three parts: the choroid, the ciliary body, and the iris. The iris is the most anterior portion of the vascular coat of the eye and is a circular vertically standing plate with a round aperture called the pupil. The pupil is

not exactly in the middle, but it is slightly displaced toward the nose. The iris plays the role of a diaphragm regulating the amount of light entering the eye.

The retina is the innermost of the three coats of the eyeball adhering to the vascular coat along its entire length until it reaches the pupil. Unlike the other coats it develops from the walls of the optic cup and according to its origin consists of two layers: the external pigmented layer and the internal layer which is the retina proper.