Explain the structure of human eye.

Explanation

The conjunctiva is a thin, protective mucous membrane found lining the outer surface of the eyeball.
The eye has two compartments, the anterior and posterior compartments. The anterior compartment has two chambers, first one lies between the cornea and iris and the second one lies bet ween the iris and lens. These two chambers are filled with watery fluid called aqueous humor. The posterior compartment lies between the lens and retina and it is filled with a jelly like fluid called vitreous humor that helps to retain the spherical nature of the eye. Eye lens is transparent and biconvex, made up of long columnar epithelial cells called lens fibres.
The eye ball is spherical in nature. The wall of the eye ball consists of three layers: fibrous Sclera, vascular Choroid and sensory Retina.
The outer coat is composed of dense non­ vascular connective tissue. It has two regions: the anterior cornea and the posterior sclera. Cornea is a non-vascular transparent coat formed of stratified squamous epithelium.
Sclera forms the white of the eye and protects the eyeball. Posteriorly the sclera is innervated by the optic nerve.
Choroid is highly vascularized pigmented layer that nourishes all the eye layers and its pigments absorb light to prevent internal reflection.
Anteriorly the choroid thickens to form the ciliary body and iris. Iris is the coloured portion of the eye lying between the cornea and lens. The aperture at the centre of the iris is the pupil through which the light enters the inner chamber.
Iris is made of two types of muscles the dilat9r papillae (the radial muscle) and the sphincter papillae (the circular muscle). In the bright light, the circular muscle in the iris contract, so that the size of pupil decreases and less light enters the eye. In dim light, the radial musle in the iris contract, so that the pupil size increases and more light enters the eye. Smooth muscle present in the ciliary body is called the ciliary muscle which alters the convexity of the lens for near and far vision. The ability of the eyes to focus objects at varying distances is called accommodation which is achieved by suspensory ligament, ciliary muscle and ciliary body. The suspensory ligament extends from the ciliary body and helps to hold the lens in its upright position. The ciliary body is provided with blood capillaries that secrete a watery fluid called aqueous humor that fills the anterior chamber.

Retina forms the inner most layer of the eye. The neural retina layer contains three types of cells: photoreceptor cells-cones and rods, bipolar cells and ganglion cells. The yellow fiat spot at the centre of the posterior region of the retina is called macula lutea which IS responsible for sharp detailed vision. A small depression present in the centre of the yellow spot is called fovea centralis which contains only cones. The optic nerves and the retinal blood vessels enter the eye slightly below the posterior pol, which is devoid of photo receptors, hence this region is called blind spot