Physiology and Hygiene for Secondary Schools

by Francis M. Walters

Fig. 146—Sense organs of taste. A. Map of upper surface of tongue, showing on the left the different kinds of papillæ, and on the right the areas of taste (after Hall). Area sensitive to bitter (——); to acid (....); to salt (—.—.—.—); to sweet (————). B. Section through a papilla. n. Small nerve connecting with taste buds at d. e. Epithelium. C. Single taste bud magnified. n. Nerve, the fibers of which terminate between the spindle-shaped cells a. e. Epithelial cells.

The Sensation of Taste.—The sense organs of taste are found chiefly in the mucous membrane covering the upper surface of the tongue. Scattered over this surface are a number of rounded elevations, or large papillæ (A, Fig. 146). Toward the back of the tongue two rows of these, larger than the others, converge to meet at an angle, where is located a papilla of exceptional size. Surrounding each papilla is a narrow depression, within which are found the sense organs of taste (B, Fig. 146). These are called, from their shape, taste buds, and each bud contains a central[pg 345] cavity which communicates with the surface by a small opening—the gustatory pore. Within this cavity are many slender, spindle-shaped cells which terminate in hair-like projections at the end nearest the pore, but in short fibers at the other end. Nerve fibers enter at the inner ends of the buds and spread out between the cells (C, Fig. 146). These fibers pass to the brain as parts of two pairs of nerves—those from the front of the tongue joining the trigeminal nerve, and those from the back of the tongue, the glossopharyngeal nerve.

The gustatary, or taste stimulus, is some chemical or physical condition of substances which is manifested only when they are in a liquid state. For this reason only liquid substances can be tasted. Solids to be tasted must first be dissolved.

[pg 346]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 (page 161).

Fig. 147—Sense organ of smell. A. Distribution of nerves in outer wall of nasal cavity. 1. Turbinated bones. 2. Branch of fifth pair of nerves. 3. Branches of olfactory nerve. 4. Olfactory bulb. B. Diagram showing connection of neurons concerned in smell.

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 (B, Fig. 147). In fact, the olfactory cells[pg 347] 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 (A, Fig. 147).

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.