Special senses and their functions
The special senses are the senses which have specialized organs dedicated to them. In this section, we discussed special senses in the human body.
1. The Eye
Of all the special senses, vision is undoubtedly the most important.
1. Light enters the eyeball through the transparent cornea.
2. It passes through the pupil which is the central gap or aperture.
3. Depending on the intensity of light, the pupil can become bigger or smaller. The size of the aperture is controlled reflexly the radial and circular fibers of the muscular sheet like curtain—the iris.
4. The iris can also contract when an effort is made to focus on a near object.
5. After passing through the transparent jelly like fluid (vitreous), light falls on an expanded sheet of nervous cells —the retina.
6. The focusing of light is done by the lens which lies just behind the pupil. When a person is young, the lens is soft and can be made more or less powerful (to a small extent) by a muscle —-the ciliary body.
7. To prevent the lens and iris from touching the cornea, there is a chamber or space filled with water like fluid —aqueous humor. Aqueous humor is being constantly formed and absorbed, i.e. it is in a state of dynamic equilibrium.
8. When a light image falls on the retina it excites the special nerve cells of the retina. The impulses pass upwards to the brain by special nerves called the visual pathway. The fibers of the visual pathway are so arranged that impulses from the two eyes are coordinated to give an integrated and stereoscopic image. The visual cortex —the part of the brain which “sees” the image —interprets the image and perceives it.
9. To prevent injury to the eye, there are the upper and lower eyelids. The eyelids have a stiff sheet-like compressed tissue, to give it a shape. It is covered with a membrane called the conjunctiva which covers its inner surface and also covers a part of the eyeball (sclera).
10. To keep the conjunctiva moist there are tear glands —lachrymal glands.
There are also glands which secrete an oil like the lubricant. The border of the eyelid has hairy eyelashes which have a protective value. The eyelids close the eye for a fraction of a second every few seconds to keep the conjunctiva moist. Also if danger threatens, the eyelids close reflexly—blinking.
Disorders of the Eye
Being a very delicate organ exposed to the outside environment, the eye is prone to many diseases, e.g.
— Corneal ulcers
Other diseases affecting the eye are
— Retinal damage due to diabetes
— Retinal damage due to high blood pressure
— Retinal detachment
In addition, injuries are common.
2. The Ear
1. The ear is made up of 3 parts:-
i) The External ear
ii) The Middle ear
iii) The Inner ear
2. The external ear is like a megaphone. It collects sound waves. The external auditory canal is covered by skin with a waxy coating. It ends at the ear drum.
3. The eardrum is the skin like a drum. It vibrates when sound waves strike it.
4. The middle ear is connected to the throat by a narrow tube –Eustachian tube which equalizes the pressure between the middle air and outside air. This allows the drum to function properly. In contact with the drum are the 3 bones of the middle ear which amplify the sound waves and pass on the vibrations to the inner ear.
5. The inner ear has 2 serrated organs:-
(a) The cochlea converts sound vibrations into nerve impulses which are conveyed by the auditory nerve (8th cranial nerve) to the brain.
(b) The vestibule and the 3 semicircular canals. These are organs which detect the position of the head in space and also movements of acceleration, deceleration, and direction. Information is conveyed to the brain by the vestibular nerve (a part of the 8th cranial nerve).