Internal Structure of eye and their function

 Internal Structure of eyeball :

The eyeball is a globular structure and only one fifth part of it is visible from outside. The internal Structure of the eyeball can be best studied by taking longitudinal section.

Structure of eyeball diagram

structure of eyeball diagram, internal structure of eyeball, internal structure of eye, physiology of Vision

Layer of eyeball :

Excepting the front portion, the wall of the eyeball is composed of three distinct concentric coats or layers. The three coats are : 

1. The Outer fibrous coat is know as Sclera.
2.  The Middle vascular coat is known as Choroid.
3.  The inner nervous coat is called retina.

(a) Sclera :

The Sclera is formed of firm, heavy, opaque, fibrous connective tissue and surrounds the entire eyeball. At the anterior portion, a part of this coat becomes transparent to form the Cornea which remains covered by Conjuctiva. The opaque portion of this coat forms the white part of the eye, which is exposed in front. The Sclera affords the shape to the eyeball and protects the inner structures and gives surface for the attachment of the extraocular muscles.

(b) Choroid :

It is a soft, thin layer and is concerned with the nutrition of the ocular structures. This layer is formed of numerous capillaries and dark brown pigment cells. The capillaries are helpful for the maintenance of the metabolism of the eye and the pigment cells provide a dark background for the light sensitive cells or the retina.

Choroid coat is subdivided in to three parts i.e

1. Choroid proper
2. Iris
3. Ciliary body

1. Choroid Proper :

It is a dark pigmented and highly vascular membrane, located between Sclera and the visual part of retina. Like the Sclerotic coat, the Choroid coat is highly modified at the front of the eyeball, forming pigmented iris and ciliary body. At the junction of Sclera and Cornea, the Choroid bends inwardly into the cavity of the eyeball in the form of circular partition, called the iris. There are pigment cells in the iris. The blue, black and brown colour of the eye is due to the colour of the iris.

2. Iris :

The iris is the anterior portion of the middle coat. It is a thin, circular, muscular, usually pigmented, centrally perforated disc like structure. It has a central circular opening, called pupil. The iris is provided with two sets of muscles i.e circular muscles and radial muscles. These muscles control the diameter of the pupil. When circular muscles contract, the size of the pupil is decreased and when radial muscles contract, the diameter is increased. In this way, the iris acts as a diaphragm and regulates accurately the amount of light entering the eyeball proper.

3. Ciliary body :

Just behind the iris and at its outer margin and in the same level is the ciliary body which consists chiefly of muscles fibres arranged in a series of tiny ridges. In it, blood vessels and glands are also present. The muscle fibres are of two types. Circular ciliary muscles and radial ciliary muscles. The ciliary body is the thickened middle portion of the middle coat. It bears the ciliary processes which give attachment to the fibres of the suspensory ligament of the lens. The adjustment of the lens usually takes place by the ciliary body.

(c) Retina :

It is the innermost coat or layer of the eyeball, which contains light receptors. It is the most sensitive part of the eye. The entire inner surface of the eyeball is lined by retina except for the anterior portion where it ends near the lens attachment. The point where the nervous region of retina ends, is called ora serrata which lies behind the ciliary body. The area where optic nerve enters, is slightly elevated is called optic disc or blind spot. In the blind spot photoreceptors cells are absent.

Internal Structure of eye and  structure of eyeball diagram.

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