Human eye have binocular vision.The eye can discriminate colour,appraise length,width and depth visually and form true inverted image.
Structure of eye
The eye are two in number and lodged in orbits(bony socket) of skull. The eye is a hollow ,spherical organ,about 2.5cm in diameter and about 6 to 8 gram in weight.It has two parts-
Eye has four protective devices.
The outwardly directed hair of the eyebrows carry the sweat and rain drops trickling down the forehead to the sides to prevent their falling into the eyes
In man two eyelids are present,upper is movable.They are regularly closed at short intervals to clean the cornea.This is called blinking. In frog out of two upper eyelid is immovable and lower eyelid is movable. Nictitating membrane is present in frog which protect eye in water . Movement of nictitating membrane tekes place by retractor bulbi.It becomes folded by levator bulbi.
A nonfunctional vestigeal nictitating membrane,called plica-semilunaris, occurs in human eyes. It remains permanently retracted at the inner angle of each eye.
The eyelids bear at the free edge a row of stiff hair, the eye lashes. These check the entry of dust particles, tiny insects and rain drops into the eyes.
a. Meibomian gland
The eye-lids bear at the free edge a row of meibomian gland that is modified sebaceous gland.(Act as a lubricant).
Lacrimal gland or Tear gland
It lies in the upper outer part of the orbit and secretes a slightly saline, watery fluid that contains a bacteriolytic enzyme name lysozyme.This secretion moistens the surface of the eyeball.The excess of this secretion passes through nasolacrimal duct.It is modified sweat gland.
Some aquatic mammals(whale) possess harderain gland which lubricate nictitating membrane.It is also found in frog and birds.
d. Gland of zeis(zis)
These are modified sebaceous gland,found at base of hair follicle of eye lashes,pour lubricating fluid in hair follicle.Infection of these glands is Sty.