Production of Sensations : Part 3

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 22 of 25)

The different taste sensations are described as bitter, sweet, sour, and saline, and in the order named are recognized as the tastes of quinine, sugar, vinegar, and salt. As to how these different tastes are produced, little is known. Flavors such as vanilla and lemon, and the flavors of meats and fruits, are really smelled and not tasted. Taste serves two main purposes: it is an aid in the selection of food and it is a means of stimulating the digestive glands.

The Sensation of Smell. - The sense organs of smell are found in the mucous membrane lining the upper divisions of the nasal cavities. Here are found two kinds of cells in great abundance - column-shaped epithelial cells and the cells which are recognized as the sense organs of smell. These olfactory cells are spindle-shaped, having at one end a slender, thread-like projection which reaches the surface, and at the other end a fiber which joins an olfactory nerve. In fact, the olfactory cells resemble closely the cell-bodies of neurons, and are thought to be such. The divisions of the olfactory nerve pass through many small openings in the ethmoid bone to connect with the olfactory bulbs, which in turn connect with the cerebrum.

The Olfactory Stimulus. - Only substances in the gaseous state can be smelled. From this it is inferred that the stimulus is supplied by gas particles. Solids and liquids are smelled because of the gas particles which separate from them. The substance which is smelled must be kept moving through the nostrils and made to come in direct contact with the olfactory cells. There is practically no limit to the number of distinct odors that may be recognized.

Value of Smell. - Although the sense of smell is not so acute in man as in some of the lower animals, it is, nevertheless, a most important and useful gift. It is the only sense that responds to matter in the gaseous state, and is, for this reason, the only natural means of detecting harmful constituents of the atmosphere. In this connection it has been likened to a sentinel standing guard over the air passages. Many gases are, however, without odor, and for this reason cannot be detected by the nostrils. It is of especial importance that gases which are likely to become mixed with the air supply to the body have odor, even though the odor be disagreeable. The bad odors of illuminating gas and of various compounds of the chemical laboratory, since they serve as danger signals to put one exposed to them on his guard, are of great protective value.

Sight and Hearing. - The sense organs of sight and hearing are highly complicated structures, and will be considered in the chapters following.

Summary. - Sensations are certain activities of the mind that result from excitations within the body or at its surface. These cause the neurons to discharge impulses which on reaching the cerebrum cause the sensations. Sensations are necessary for intelligent and purposeful action and for acquiring all kinds of knowledge. To enable the stimuli to act to the best advantage in starting the impulses, special devices, called sense organs, are employed. These receive the terminations of the neurons, and by their special structure enable the most delicate stimuli to start impulses. The simpler forms of sense organs are those of touch, temperature, taste, and smell.

Exercises. - 1. Compare sensations and reflex actions with reference to their nature and cause. Give steps in the production of each.

2. Give examples of sensation stimuli. State the purpose of sense organs.

3. How do general sensations differ from special sensations?

4. Of what value is pain in the protection of the body?

5. Show that sensations lead to the higher forms of mental activity, such as emotion and imagination.

6. Of what value to the body is the "localizing of the sensation"?

7. What kinds of sense organs are found in the skin? State the purpose of each.

8. Through what sense avenues is one made aware of solids, of liquids, and of gases?

9. Of what special protective value is the sense of smell?

Practical Work

To demonstrate the Pacinian Corpuscles. - Spread out the mesentery from the intestine of a cat and hold it between the eye and the light: Pacinian corpuscles will appear as small translucent bodies having the general form of grains of wheat. Secure a portion of the mesentery over a circular opening in a thin piece of cork and examine it with a microscope of low power. Follow the course of the nerve fiber to the nerve from which it branches.

To show Relative Sensitiveness of Different Parts of the Skin. - Holding a bristle between the fingers, bring the end in contact with the skin, noting the amount of pressure necessary to cause a sensation of touch. Test the lips, tongue, tips of fingers, and palm and back of hand, trying different sizes of bristles. Has the degree of sensitiveness any relation to the thickness of the cuticle?

To show Perceptive Differences of Different Portions of the Skin. - Place the points of a pair of dividers on the back of the hand of one who looks in the opposite direction. Is one point felt or two? Repeat several times, changing the distance between the points until it is fully determined how near the two points must be placed in order to be felt as one. In like manner test other parts of the body, as the tips of the fingers and the back of the neck. Compare results obtained at different places.

To locate Warm and Cold Sensation Spots. - Slowly and evenly draw a blunt-pointed piece of metal over the back of the neck. If it be of the same temperature as the skin, only touch sensations will be experienced. If it be a little colder (the temperature of the room) sensations of cold will be felt at certain spots. If slightly warmer than the body, heat sensation spots will be found on other parts of the skin. If the heat and cold sensation spots be marked and tested from day to day they will be found to remain constant as to position. Inference.

D.C. Heath and Co. - Publishers
Original copyright 1909