What is vision? Describe vision in Tetrapods. Give structure of Human eye

Vision (Photoreception):

It is the primary sense that vertebrates in a light filled environment use and consequently their photoreceptive structures are well developed. Most vertebrates have eyes capable of forming visual images.

Amphibia:
They are primarily sight feeders. A number of adaptations allow the eyes of amphibians to function in terrestrial environments. The eyes of some amphibians (e.g. anurans, salamanders) are close together on the front of the head and provide binocular vision and well developed depth perception necessary for capturing prey. Some amphibians with smaller and more lateral eyes (some salamanders) lack binocular vision. Their more laterally compressed eyes permit these animals to see well off to their sides. The transparent nictitating membrane (an inner eyelid) is movable and cleans and protects the eye.

Reptiles:

Vision is the dominant sense in most reptiles, and their eyes are similar to those of amphibians. Upper and lower eyelids, a nictitating membrane and a blood sinus protect and clean the surface of the eye. In snakes and some lizards upper and lower eyelids fuse in the embryo to form a protective window of clear skin called the spectable.

Some reptiles possess a median (parietal) eye that develops from outgrowths of the roof of the optic tectum (midbrain). In Tantara the median eye is complete with a lens, nerve and retina. In other reptiles median eye is less developed. Skin covers median eyes which probably can not form images. They can differentiate light and dark periods and are used in orientation to sun.

Birds:

Vision is important sense for most birds. Structure of bird eye is similar to that of other vertebrates. Birds have a unique double focusing mechanism. Pad like structures control the curvature of the lens and cilary muscles change the curvature of cornea. Double focusing allows the bird of prey to focus on a fish throughout a brief but breathtakingly fast, descent. Like reptiles birds have a nictitating membrane that is drawn over the eye ball surface to cleanse and protect it.

In all vertebrates retina is well developed. Its basement layer is composed of pigmented epithelium that covers the choroid layer. Nervous tissue that contains photoreceptors lies on this basement layer. Photoreceptors are called rods and cone cells because of their shape. Rods are sensitive to dim light whereas cones respond to high intensity light and are involved in colour perception.

Working:

When a pigment (rhodopsin) in a red cell absorbs light energy, the energy that this reaction releases triggers the generator potential in an axon and then an action potential that leaves the eye ball via the optic nerve. When the photoreceptor cells are not being stimulated (i.e. in the dark), vitamin A and energy from ATP convert rhodopsin back to its light sensive form.

Structure of Human eye:

The eye ball has a lens, a sclera (tough outer coat) a choroid layer (a thin middle layer) and an inner retina containing many light sensitive receptor cells (photoreceptors). The transparent cornea is continuous with sclera and covers the front of the eyeball. Choroid tissue also extends to the front of the eyeball to form the iris, ciliary body and suspensory ligaments. The coloured iris is heavily endowed with light screening pigments and it has radial and circular smooth muscles for regulating the amount of light entering the pupil. A clear fluid aqueous humour fills the anterior and bosterior chambers which lie between the lens and cornea. The lens is behind the iris and a jelly like vitreous body fills the vitreous chamber behind the lens. The moist mucous membrane that covers the eyeball is conjunction.