Especially in the family, and other groups of closely a.s.sociated individuals, should the nervous effect of one member upon the others be considered and every effort made to secure and maintain harmonious relations.

*The High Ideal.*-Everything considered, the conditions most favorable to the healthfulness of the nervous system are in harmony with what our greatest teachers have pointed to as the higher plane of living. On this account a true conception of the value and meaning of life is of the greatest importance. _An ever present, strong desire to live a vigorous, but simple and n.o.ble, life_ will suggest the proper course to pursue when in doubt and will stimulate the power of self-control. It will lead to the stopping of "nerve leaks" and to the maintenance of harmonious relations with one's fellows. It will cause one to recoil from the use of alcohol and other nerve poisons, as from a deadly serpent, seeing the end in the beginning, and will be the means eventually of leading the body into its greatest accomplishments.

*Summary.*-The nervous system, on account of its delicate structure, is liable to injury through wrong methods of using it and also through the introduction of drugs, or poisons, into the body. There are also found in our methods of living and systems of education conditions that tend to waste the nervous energy. To protect the nervous system from all these threatened dangers requires, among other things, the power of self-control. This enables the individual to direct his life according to his highest ideals and to free himself from habits known to be injurious.

Children must have their nervous systems safeguarded by parents and teachers. Especially must they be kept from becoming enslaved to some drug, such as alcohol or the nicotine of tobacco.

*Exercises.*-1. In what respect is the hygiene of the nervous system the hygiene of the entire body?

2. Of what value in the hygiene of the nervous system is the power of self-control? How is the habit of self-control formed?

3. Name several forms of activity that waste the nervous energy.

4. Name several influences that react unfavorably on the nervous systems of children.

5. How may too much reading prove injurious to the nervous system?

6. What forms of physical exercise are beneficial to the brain worker?

7. Why is the use of alcohol even in small quant.i.ties to be regarded as a dangerous practice?

8. Name several causes of nervousness.

9. What are the unanswerable arguments for preventing the use of tobacco by the young?

10. Why do cigarettes have a more harmful effect upon the body than other forms of tobacco?

11. Enumerate conditions in the schoolroom that dissipate the nervous energy of pupils; that economize it.

CHAPTER XX - PRODUCTION OF SENSATIONS

Our study of the nervous system has shown that impulses arising at the surface of the body are able, through connecting neurons, to bring about various activities. Moving along definite pathways, they induce motion in the muscles, and in the glands the secretion of liquids. It is now our purpose to consider the effect produced by afferent impulses upon the brain and, through the brain, upon the mind.(114) This effect is manifested in a variety of similar forms, known as

*The Sensations.*-Sensations const.i.tute the lowest forms of mental activity. Roughly speaking, they are the states of mind experienced as the _direct_ result of impulses reaching the brain. In a sense, just as impulses pa.s.sing to the muscles cause motion, impulses pa.s.sing to the brain cause sensations. The feeling which results from the hand's touching a table is a sensation and so also is the pain which is caused by an injury to the body. The mental action in each case is due to impulses pa.s.sing to the brain. Care must be exercised by the beginner, however, not to confuse sensations with the nervous impulses, on the one hand, or with _secondary_ mental effects, such as emotion or imagination, on the other.

Sensations are properly regarded as the first conscious effects of the afferent impulses and as the _beginning stage_ in the series of mental processes that may take place on account of them.

In some way, not understood, the mind a.s.sociates the sensation with the part of the body from which the impulses come. Pain, for example, is not felt at the brain where the sensation is produced, but at the place where the injury occurs. This a.s.sociation, by the mind, of the sensations with different parts of the body, is known as "localizing the sensation."

*Sensation Stimuli.*-While the sensations are dependent upon the afferent impulses, the afferent impulses are in turn dependent upon causes outside of the nervous system. If these are removed, the sensations cease and they do not start up again unless the exciting influences are again applied.

Any agency, such as heat or pressure, which, by acting on the neurons of the body, is able to produce a sensation, may be called a _sensation stimulus_. It has perhaps already been observed that the stimuli that lead to voluntary action, as well as those that produce reflex action of the muscles, cause sensations at the same time. From this we may conclude that sensation stimuli are the same in character as those that excite motion.

On the other hand, it should be noted that sensations are constantly resulting from stimuli that are of too mild a nature to cause motion.

*Cla.s.ses of Sensations.*-Perhaps as many as twenty distinct sensations, such as pain, hunger, touch, etc., are recognized. If these are studied with reference to their origin, it will be seen that some of them result from the action of definite forms of stimuli upon the neurons terminating in sense organs; while the others, as a rule, arise from the action of indefinite stimuli upon neurons in parts of the body that do not possess sense organs. The members of the first cla.s.s-and these include the sensations of touch, temperature, taste, smell, hearing, and sight-are known as the _special_ sensations. The others, including the sensations of pain, hunger, thirst, nausea, fatigue, comfort, discomfort, and those of disease, are known as _organic_, or general, sensations. These two cla.s.ses of sensations differ in their purpose in the body as well as in the manner of their origin.

*Purposes of Sensations.*-Any given sensation is related to the stimulus which excites it as an _effect_ to a _cause_. It starts up or stops, increases in intensity or diminishes, according to the action of the exciting stimulus. As the stimuli are outside of the nervous system, and in the majority of cases outside of the body, the sensations indicate to the mind what is taking place either in the body itself or in its surroundings. They supply, in other words, the means through which the mind acquires information. By means of the special sensations, a knowledge of the physical surroundings of the body is gained, and through the organic sensations the needs of the body and the state of the various organs are indicated. In general, sensations are made to serve two great purposes in the body, as follows:

1. They provide the necessary conditions for intelligent and purposeful action on the part of the body.

2. They supply the basis for the higher mental activities, as perception, memory, thought, imagination, and emotion.

Intelligent action is impossible without a knowledge both of the bodily organs and of the body's surroundings. Protection and the regulation of the work of an organ necessitate a knowledge of its condition, while the adapting and adjusting of the body to its surroundings require a knowledge of what those surroundings are. The dependence of all the higher forms of mental activity upon sensations is recognized by psychologists and is easily demonstrated by a study of the manner in which we acquire knowledge. "Without sensation there can be no thought."

*Steps in the Production of Sensations.*-The steps in the production of sensations are not essentially different from those in the production of reflex action. First of all, external stimuli act upon the fiber terminations in the sense organs, or elsewhere, starting impulses in the neurons. These pa.s.s into the central nervous system and there excite neurons which in turn discharge impulses into the cerebrum. The result is to arouse an activity of the mind-a sensation. The steps in the production of any _special_ sensation naturally involve the following parts:

1. A sense organ where the terminations of the neurons are acted upon by the stimulus.

2. A chain of neurons which connect the sense organ with the brain.

3. The part of the cerebrum which produces the sensation.

*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 particular 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 papillae of the skin (Fig.

144). They are composed mainly of connective tissue. Each corpuscle receives the termination of one or more nerve fibers. These, on entering, lose the medullary sheath and separate into a number of branches that penetrate the corpuscle in different directions.

[Fig. 144]

Fig. 144-*A touch corpuscle* highly magnified. (See text.)

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. (See Practical Work.) The Pacinian corpuscles are composed of connective tissue arranged in separate layers around a narrow central cavity called the core (Fig. 145).

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.

[Fig. 145]

Fig. 145-*Pacinian corpuscle*, magnified. _A._ Medullated nerve fiber.

_B._ Axis cylinder terminating in small bulb at _C._ _D._ Concentric layers of connective tissue. _E._ Inner bulb.

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. (See Practical Work.)

*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.

[Fig. 146]