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.

Comments

Post a Comment

Popular posts from this blog

Compare the anatomy of Bifacial and Isobilateral leaves

BIFACIAL LEAF: It is also called dorsiventral leaf. This leaf is more strongly illuminated on upper surface than on lower surface. This unequal illumination induces a difference in the internal structure between upper and lower sides. Its internal structure shows: (1)        Upper Epidermis This is single layer of cells with thick cuticle which checks excessive evaporation of water from the surface. It does not contains chloroplasts, stomata are also absent. (2)        Lower Epidermis This is also single layer but with a thin cuticle. It bears many stomata, two guard cells of which contain some chloroplast, none are present in the epidermal cells. Internal to each stomata is large cavity called respiratory cavity. Lower epidermis of leaf is meant for exchange of gases (oxygen and carbon dioxide) between the atmosphere and the plant body. Excess water also evaporates from lower epidermis. (3)...
Read more

Autobiographical elements in A Farewell to Arms

Introduction Some, literature can be satisfactorily read and discussed taking the author into account. Other literature seems inseparable from the person who created it. To an extraordinary degree Hemingway and what he has written exist in a synergetic relationship, reinforcing and fulfilling each other; he has created a personal legend which serves as an ambience in which we read him. Autobiographical Touches The novel A Farewell to Arms autobiographical elements. The love-affair between Fredric and Barkley, the hero’s injury in the battlefield, his despair and frustration, the hero’s separate peace with life are some of the characteristics of Hemingway’s life. Hemingway himself had received the injury caused by the mortar-shell at Fossalta, and he was admitted into the Milan hospital for recuperation in the late summer and autumn of 1918. He fell in love with the nurse in the Milan hospital. Carlos Baker has thrown light on this fact. In A Farewell to Arms Hemingway was d...
Read more

osmoregulation in terrestrial and aquatic animals

Mechanism of osmoregulation is very important for all groups of animals whether inhibting land or water. Unlike plant cells, animal cells when placed in hypotonic solution burst due to the continuous absorption of water. On the contrary they would shrink and die if constantly placed in hypertonic solution. Normally uptake and loss of water are in balance for proper survival of cell. (1)        Osmoregulation in terrestrial animals: Terrestrial animals are more likely to loose water by evaporation through their permeable surfaces exposed to amsophere. Among various animal groups only Arthropods and vertebrates became the most successful land dwellers. They have developed number of strategies to maintain osmoregulation of their body fluid. (a) Water proof external coverings: To prevent water loss through external surfaces, vertebrates like reptiles, birds and mammals have water proof keratinized epidermis. Similarly the insects have developed...
Read more