The transparent thin membrane covering the cornea is called conjunctiva. It protects the eyes from dust and germs. It keeps the eyes slippery and moist.

(i) Upper eyelid: The upper lid of the eye is called upper eyelid. Under it there is a line of eye lashes like hair. It is contagious. It controls the opening and closing of the eyes.
(ii) Inferior eyelid: The lower eyelid is called lower eyelid. Its free end has single eye lashes like hair. It is not contagious. It protects the eyes from dust and sand.
(iii) Nectitating screen: It is the hidden almost passive part. It is located in the inner corner of both eyes. It is a reddish flesh. Through this the lacrimal gland ducts are opened.

1. Helps light rays enter the eye.
2. Regulates internal and external eye pressure.
3. Through it the light rays are refracted and fall on the retina.

The large chamber between the lens and the retina is called the vitreous chamber. It is 80% of the eyeball. It is filled with a jelly-like fluid called vitreous humor. Vitreous humor is like egg white. Vitreous humor consists of 99% water, 1% collagen and hyaluronic acid. Vitreous humor helps in refraction of light.

The cavity between iris and lens is called posterior chamber. It is also filled with aqueous humor.

The cavity between the cornea and the lens is called the aqueous humor chamber. It is filled with a fluid called aqueous humor.

The eyeball has three cavities filled with fluid.
(i) Aqueous humor chamber: The cavity between the cornea and the lens is called the aqueous humor chamber. It is filled with a fluid called aqueous humor.
(ii) Posterior chamber: The cavity between iris and lens is called posterior chamber. It is also filled with aqueous humor.
(iii) Vitreous humor chamber: The large chamber between the lens and the retina is called the vitreous chamber. It is 80% of the eyeball. It is filled with a jelly-like fluid called vitreous humor. Vitreous humor is like egg white. Vitreous humor consists of 99% water, 1% collagen and hyaluronic acid. Vitreous humor helps in refraction of light.

A lens is attached to the ciliary body by the suspensory ligament behind the pupil. It is transparent, elastic and bi-convex. Its diameter is 10 mm and thickness is 3.7-4.5 mm. There is no blood supply to the lens. Contraction-dilation of the ciliary muscle causes the lens to contract-dilate. With the help of the lens, the light rays coming from the object are reflected to certain parts of the retina.

(i) Retina produces reflections.
(ii) The bile duct forms a clear reflex.
(iii) The optic nerve sends messages to the brain.

Above the optic disc in the retina is an oval and yellow area, called the macula lutea. The small hole in the center of the macula lutea is called the phobia centralis or yellow spot. It has very few rod cells and a lot of angle cells, so the best reflection is formed.