Production of Sensations : Part 2

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 21 of 25)

Sense Organs. - The sense organs are not parts of the afferent neurons, but are structures of various kinds, in which the neurons terminate. Their function is to enable the sensation stimuli to start the impulses. By directing, concentrating, or controlling the stimuli, the sense organs enable them to act to the best advantage upon the neurons. When it is recognized that such widely different forces as light waves, sound waves, heat, pressure, and odors are enabled by them to stimulate neurons, the importance of these organs becomes apparent. As would naturally be inferred, the construction of any sense organ has particulary reference to the nature of the stimulus which it is to receive. This is most apparent in the sense organs of sight and hearing.

Simple Forms of Sense Organs. - The simplest form of a sense organ (if such it may be called) is one found among the various tissues. It consists of the terminal branches of nerve fibers which spread over a small area of cells, as a network or plexus. Such endings are numerous in the skin and muscles.

Next in order of complexity are the so-called end-bulbs. These consist of rounded, or elongated, connective tissue capsules, within which the nerve fibers terminate. On the inside the fibers lose their sheaths and divide into branches, which wind through the capsule. End-bulbs are abundant in the lining membrane of the eye, and are found also in the skin of the lips and in the tissues around the joints.

Slightly more complex than the end-bulbs are the touch corpuscles. These are elongated bulb-like bodies, having a length of about one three-hundredth of an inch, and occupying the papilla of the skin. They are composed mainly of connective tissue. Each corpuscle receives the termination of one or more nerve fibers. These, on entering, lose the medullar sheath and separate into a number of branches that penetrate the corpuscle in different directions.

The largest of the simple forms of sense organs are bodies visible to the naked eye and called, from their discoverer Pacini, the Pacinian corpuscles. They lie along the course of nerves in many parts of the body, and have the general form of grains of wheat. The Pacinian corpuscles are composed of connective tissue arranged in separate layers around a narrow central cavity called the core. Within the core is the termination of a large nerve fiber. These corpuscles are found in the connective tissue beneath the skin, along tendons, around joints, and among the organs of the abdominal cavity.

The simple forms of sense organs have a more or less general distribution over the body, and are concerned in the production of at least three special sensations. These are touch, temperature, and the muscular sensation.

Touch, or feeling, is perhaps the simplest of the sensations. The sense organs employed are the touch corpuscles, and the external stimulus is some form of pressure or impact. Pressure applied to the skin, by acting on the fiber terminations in the corpuscles, starts the impulses that give rise to the sensation. The touch corpuscles render the fiber terminations so sensitive that the slightest pressure is able to arouse sensations of touch. It is found that a change of pressure, rather than pressure that is constant, is the active stimulus. That all parts of the skin are not equally sensitive to pressure, and that the mind does not interpret equally well the sensations from different parts, are facts easily demonstrated by experiment.

The Temperature Sensation. - Temperature sensations, like those of touch, are limited almost entirely to the skin. They are of two kinds, and are designated as heat sensations and as cold sensations. Whether the sense organs for temperature are different from those of touch is not known. It is known, however, that the same corpuscles do not respond alike to heat, cold, and pressure.

A Change of Temperature, rather than any specific degree of heat or cold, is the active temperature stimulus. The sensation of warmth is obtained when the temperature of the skin is being raised, and of cold when it is being lowered. This explains why in going into a hallway from a heated room one receives a sensation of cold, while in coming into the same hallway from the outside air he receives a sensation of warmth. It is for the same reason that we are able to distinguish only the relative, not the actual, temperature of bodies.

Muscular Sensations. - These are sensations produced by impulses arising at the muscles. Such impulses originate at the fiber terminations which are found in both the muscles and their tendons. By muscular sensations one is conscious of the location of a contracting muscle and of the degree of its tension. They also make it possible to judge of the weight of objects.

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 papille. 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. These are called, from their shape, taste buds, and each bud contains a central 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. 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.

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