It is through the sense organs chiefly eyes, ears, nose, mouth (tongue), and skin that the brain comes in contact with the reality of the external environment which human beings and other living creatures inhabit. Without senses vision, hearing, touch, smell, taste and others a human being as such can hardly be said to exist.
Perception of reality finally takes place in the brain, but the impressions which make this possible come via the nerve pathways of the sensory organs. The brain "dwells in utter darkness" in the cavity of the skull; but it is the brain that sees, hears, feels, and makes interpretations of all the sensory impressions brought to it.
Here we will place emphasis on the principal sense organs. There are several practical reasons for this. Eyes, ears, and skin particularly are complex and specialized organ systems, each of which demands its own specialized kind of care and attention. Furthermore, all the sensory organs (with the exception of the eye) have functions beyond reporting sense impressions to the brain. For a complete understanding of the human organism, these specialized systems deserve to be studied in their own right. They are often abused.
Eyes and ears are two of the greatest assets a human being possesses. Yet many people fail to make the most of them. Eye specialists (ophthalmologists) constantly encounter people with faulty vision who have accepted the blurred, imperfect images of the world they half-see as perfectly normal. "Oh, I see fine," they say until they surprisingly discover how much better they can see when their visual defects have been properly corrected. Ear specialists (otologists) similarly report that patients have usually lost a full third of their hearing before they take any steps to overcome their partial deafness.
End Organs of the Peripheral
To provide a more exact picture of the operation of sensory organs, we must briefly describe the operation of the peripheral nervous system. As its name implies, it is located at the periphery or outside ends of the nervous systems. It consists primarily of nerve trunks, attaching to the central and autonomic nervous systems, and end-plates, or end-organs. These end-organs pick up specific types of stimuli (light, sound, heat, cold, pain,
Pressure,etc. They also deliver back message or orders to the organs to which they are attached. End plates that pick up sensations are called receptors. Those that deliver orders are effectors. The end plates are specific for their particular sensations. Thus, the eye reacts to light, the ear to sound, the nose and tongue to chemicals in solution.
The specificity of the end plates explains some peculiar reactions. Why do you sometime see stars when you get a punch in the eye? Because the receptors server only one function. The end plates on the retina of the eye connect directly with the visual center in the brain. Stimulating the end plates of the retina with a powerful blow can be translated by the brain only in visual images that is, the flashes of light we describe as seeing stars. Similarly a blow on the ear may come as an explosive sound.
Man has far more than the traditional five senses. There are end organs for hot and cold sensations, for pressure, for pain and combinations of impacts on end organs that give a sense of vibration, a feeling of fullness or tension, a sense of balance, even the most basic feelings of hunger, thirst, and sexual desire. The end organ for hot and cold are irregulary distributed on the skin and in the mouth and esophagus, there are many of them of the feet and very few on the chest. The designers of women‘s fashions, though not deliberately, abide by these hot and cold spots.