Welcome to my blog,today we are going to discuss in details about eye or sight and after this content. Readers should be able to describe the gross structure of the eye, describe the route taken by nerve impulses from the retina to the cerebrum,explain how light entering the eye is focused on the retina and also state the functions of the extraofular eye muscles.

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What Is It About The Eye

In this section, we’re discussing about eye and how it functions .About The eye is the organ of sight, It is situated in the orbital cavity and supplied by the optic nerve(and cranial nerve),vitys almost spherical in shape and about 2.5 cm %) diameter. The space between
the eye and the orbital cavity is occupied by adipose tissue.About the eye bony walls or the orbit and the fat protect the eye from injury. Structurally the two eyes are separate but, unlike the ears, some of their activities are coordinated so that they normally function as a pair. It is possible to see with only one eye (monocular vision), but three-dimensional vision is impaired when only one eye is used, especially in relation to the judgement of speed and distance.

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Structure

About eye. There are three layers of tissue in the walls of the eye:

• The outer fibroid layer: sclera and cornea
• the middle vascular layer or uveal tract consisting of the choroid ,ciliary body and

Structures inside the eye ball include the lens, aqueous fluid and vitreous body.

Sclera and cornea

About eye,The sclera, or white of the eye, forms the outermost layer of the posterior and lateral aspects of the eyeball and is continuous anteriorly with the cornea. It consists of a firm
ribrous membrane that maintains the shape of the eye and gives attachment to the extrinsic muscles of the eye. Anteriorly the sclera continues as a clear transparent epithelial membrane, the cornea. Light rays pass through the cornea to reach the retina. The cornea is convex anteriorly and is involved in refracting bending) light rays to focus them on the retina.

Choroid

About eye,The choroid lines the posterior five-sixths of the inner surface of the sclera. It is very rich in blood vessels and is deep chocolate brown in colour. Light enters the eye through the pupil, stimulates the sensory receptors in the retina and is then absorbed by the choroid.

Ciliary body

The ciliary body is the anterior continuation of the choroid consisting of ciliary muscle (smooth muscle fibres) and secretory epithelial cells. As many of the smooth muscle fibres are circular, the ciliary muscle acts like a sphincter. The lens is attached to the ciliary body by radiating suspensory ligaments, like the spokes of a wheel.
Contraction and relaxation of the ciliary muscle fibres, Which are attached to these ligaments, control the size and thickness of the lens.
The epithelial cells secrete aqueous fluid into the anterior segment of the eye, i.e. the space between the lens and the cornea (anterior and posterior chambers). The ciliary body is supplied by parasympathetic branches of the loculomotor nerve (3rd cranial nerve). Stimulation
causes contraction of the ciliary muscle and accommodation of the eye.

Iris

About eye. The iris is the visible coloured ring at the front of the eye and extends anteriorly from the ciliary body, lying behind the cornea and in front of the lens. It divides the anterior segment of the eye into anterior and posterior chambers which contain aqueous fluid secreted by the ciliary body. It is a circular body composed of pigment cells and two layers of smooth muscle fibres, one circular and the other radiating . In the centre is an aperture called the pupil. The iris is supplied by parasympathetic and sympathetic nerves. Parasympathetic stimulation constricts the pupil and sympathetic stimulation dilates it.
The colour of the iris is genetically determined and depends on the number of pigment cells present. About eye of Albinos have no pigment cells and people with blue eyes have fewer than those with brown eyes.

Lens

The lens is a highly elastic circular biconvex body, lying immediately behind the pupil. It consists of fibres enclosed within a capsule and is suspended from the ciliary bodyby the suspensory ligament. Its thickness is controlled by the ciliary muscle through the suspensory ligament. The lens bends (refracts) light rays reflected by objects in front of the eye. It is the only structure in the eye that can vary its refractory power, which is achieved by changing its thickness. When the ciliary muscle contracts, it moves forward, releasing its pull on the lens, increasing its thickness. The nearer is the object being viewed, the thicker the lens becomes to allow focusing.

Retina

About eye .The retina is the innermost lining of the eye .It is an extremely delicate structure and well adapted for stimilation by light rays. It is composed of several layers of nerve cell bodies and their axons, lying on a pigmented layer of the epithelial cells.