Receptors In Man – The third and the inner most layer of eye is the retina. It is the sensitive layer of the eye. It is supplied by branches of optic nerve which enters the eye ball from brain.
The sensory cells of retina are of two types:
- Rods 2. Cones
These cells help in the formation of image on retina. The cones enable to differentiate color while the rods are sensitive to white light. Above the point of entry of optic nerve is a blind spot.
(Animals like cat can see in almost dark as it has additional rods but cannot differentiate colors because cones are absent from its retina.)
The iris divides the cavity or eye into an anterior aqueous chamber containing a fluid the aqueous humour and a large posterior chamber, the vitreous chamber; this contains transparent jelly like vitreous humour. The pupil (opening in the iris), the lens behind the iris, the aqueous humour and the vitreous humour are all involved in the focusing of a clear and sharp image on retina.
(Study the model of human eye and a fresh eye of a sheep / goat.)
There are six muscles present in the orbit attached to the outer side of sclerotic layer that cause the movements of the eye in different directions. Also present in the sockets are the tear glands, the secretion of which washes the eyes.
Image Formation
During focusing, the light rays coming form object pass through cornea and lens. The light rays reflected from a given point at a distance striking the cornea are almost parallel to each other. These rays are focused by the lens on the retina on a special spot called fovea centralis (yellow spot) fro producing sharp image.
The lens plays its role by changing its convexity to produce enough refraction in addition to that produced by cornea to bring to image into focus and from a clear image on retina.
Farsighted and nearsighted vision
Farsightedness
In far or long sightedness the image is formed behind the retina. The cornea and the lens cannot bend the light rays enough to focus on the retina. Artificial lenses are used that bend the incoming light rays before they reach cornea thus aiding the cornea and lenses to focus the image on retina.
Nearsightedness
In the near or short sightedness (myopia), the lens and cornea focus the image of a distant object at a point in front of the retina. This problem is corrected by a concave lens which is thinner in the center than at the edges.
Colorblindness
As already mentioned there are two kinds of light receptor cells in the retina, the rods and the cones. These are the cones that enable us to distinguish colors. There are three kinds of cone cells adapted to receive three primary colors i.e. blue, green and red. To receive other colors the eye requires coordinated activation of the three kinds of its cone cells. In a colorblind person one type of cone cells are lacking. Such a person is unable to distinguish between green and red colors and is called colorblind. Colorblindness is a genetic (hereditary) defect (Fig. 15.6), discussed in detail in chapter 17.
Fig 15.6 color blindness chart
(There are certain other defects of human eye such as night blindness in which a person is unable to see after sun-set; and cataract which is the defect of aqueous humour.)