CHAPTER XVIII - PHYSIOLOGY OF THE NERVOUS SYSTEM
In the preceding chapter was pointed out the method by which the different parts of the body are brought into communication by the neurons or nerve cells. We are now to study the means whereby the neurons are made to control and coördinate the different parts of the body and bring about the necessary adjustment of the body to its surroundings. This work of the neurons naturally has some relation to their properties.
Properties of Neurons.—The work of the neurons seems to depend mainly upon two properties—the property of irritability and the property of conductivity. Irritability was explained, in the study of the muscles (page 243), as the ability to respond to a stimulus. It has the same meaning here. The neurons, however, respond more readily to stimuli than do the muscles and are therefore more irritable. Moreover, they are stimulated by all the forces that induce muscular contraction and by many others besides. They are by far the most irritable portions of the body.
Conductivity is the property which enables the effect of a stimulus to be transferred from one part of a neuron to another. On account of this property, an excitation, or disturbance, in any part of a neuron is conducted or carried to all the other parts. Thus a disturbance at the distant ends of the dendrites causes a movement toward the cell-body and, reaching the cell-body, the disturbance is[pg 305] passed through it into the axon. This movement through the neuron is called the nervous impulse.
Purpose of the Impulse. —Though the nature of the nervous impulse is not understood, 103 its purpose is quite apparent. It is the means employed by the nervous system for controlling and coördinating the different parts of the body. The arrangement of the neurons enables impulses to be started in certain parts of the nervous system, and the property of conductivity causes them to be passed as stimuli to other parts. This enables excitation at one place to bring about action at another place.
Acting as stimuli, the impulses seem able to produce two distinct effects: first, to throw resting organs into action and to increase the activity of organs already at work; and second, to diminish the rate, or check entirely, the activity of organs. Impulses producing the first effect are called excitant impulses; those producing the second effect, inhibitory impulses.
Functions of the Parts of Neurons.—The cell-body serves as a nutritive center from which the other parts derive nourishment. Proof of this is found in the fact that when any part of the neuron is separated from the cell-body, it dies, while the cell-body and the parts attached to the cell-body[pg 306] may continue to live. In addition to this the cell-body probably reënforces the nervous impulse.
The dendrites serve two purposes: first, they extend the surface of the cell-body, thereby enabling it to absorb a greater amount of nourishment from the surrounding lymph; second, they act as receivers of stimuli from other neurons. The same impulse does not pass from one neuron to another. An impulse in one neuron, however, is able to excite the neuron with which it makes an end-to-end connection, so that a series of impulses is produced along a given nerve path (Fig. 129).
The special function of the axon is to transmit the impulse. By its length, structure, and property of conductivity it is especially adapted to this purpose. The axis cylinder, however, is the only part of the axon concerned in the transmission. The primitive sheath and the medullary layer protect the axis cylinder, and, according to some authorities, serve to insulate it. The medullary sheath may also aid in the nourishment of the axis cylinder.
Nerve Stimuli.—While the properties of irritability and conductivity supply a necessary cause for the production and transmission of nervous impulses, these alone are not sufficient to account for their origin. An additional cause is necessary—a force not found in the nerve protoplasm, but one which, by its action on the protoplasm, makes it produce the impulse. In this respect, the neuron does not differ essentially from the cell of a muscle. Just as the muscle cell requires a stimulus to make it contract, so does the neuron require a stimulus to start the impulse. Hence, in accounting for the activities of the body, it is not sufficient to say they are caused by nervous impulses. We must also investigate the nerve stimuli—the means through which the nervous impulses are started. Most of these[pg 307] are found outside of the body and are known as external stimuli.
Action of External Stimuli.—In the arrangement of the nervous system the most favorable conditions are provided for the reception of external stimuli. Not only do vast numbers of neurons terminate at the surface of the body,104 but they connect there with delicate structures, called sense organs. The purpose of the sense organs is to sensitize (make sensitive) the terminations of the neurons. This they do by supplying special structures through which the stimuli can act to the best advantage upon the nerve endings. Moreover, there are different kinds of sense organs, and these cause the neurons to be sensitive to different kinds of stimuli. Acting through the sense organs adapted for receiving them, light, sound, heat, cold, and odors all act as stimuli for starting impulses. Indeed, the arrangement is so complete that the nervous system is subjected to the action of external stimuli in some form practically all the time. The work of the sense organs is further considered in Chapters XX, XXI, and XXII.
How External Stimuli act on Internal Organs.—For stimulating the neurons not connected with the body surface we are dependent, so far as known, upon the nervous impulses. An impulse started by the external stimulus goes only so far as its neuron extends. But it serves as a stimulus for the neuron with which the first connects and starts an impulse in this connecting neuron, the point of stimulation being where the fiber terminations of the first neuron make connection with the dendrites of the second. This impulse in turn stimulates the next neuron, and so on, producing a series of impulses along a given nerve path. [pg 308]In this way the effect of an external stimulus may reach and bring about action in any part of the body. This is in brief the general plan of inducing action in the various organs of the body. This plan, however, is varied according to circumstances, and at least three well-defined forms of action are easily made out. These are known as reflex action, voluntary action, and secondary reflex action.
Reflex Action.—When some sudden or strong stimulus acts upon the nerve terminations at the surface of the body, an immediate response is frequently observed in some quick movement. The jerking away of the hand on accidentally touching a hot stove, the winking of the eyes on sudden exposure to danger, and the quick movements from slight electrical shocks are familiar examples. The explanation of reflex action is that external stimuli start impulses in neurons terminating at the surface of the body and these, in turn, excite impulses in neurons which pass from the spinal cord or brain to the muscles (Fig. 138). Since there is an apparent turning back of the impulses by the cord or brain, the resulting movements are termed reflex.105
Fig. 138—Diagram illustrating reflex action of an external organ.
Reflex Action and the Mind.—If one carefully studies the reflex actions of his own body, he will find that they[pg 309] occur at the time, or even a little before the time, that he realizes what has happened. If a feather is brought in contact with the more sensitive parts of the face of a sleeping person, there is a twitching of the skin and sometimes a movement of the hand to remove the offending substance. Surgeons operating upon patients completely under the influence of chloroform, and therefore completely unconscious, have observed strong reflex actions. These and other similar cases indicate clearly that reflex action occurs independently of the mind—that the mind neither causes nor controls it. If a further proof of this fact were needed, it is supplied by experiments upon certain of the lower animals,106 which live for a while after the removal of the brain. These experiments show that the nervous impulses that produce reflex action need only pass through the spinal cord and do not reach the cerebrum, the organ of the mind.
The Reflex Action Pathway.—By study of the impulses that produce any reflex action, a rather definite pathway may be made out, having the following divisions:
1. From the surface of the body to the central nervous system (usually the spinal cord). This, the afferent division, is made up of di-axonic neurons, and these have (in the case of the spinal nerves) their cell-bodies in the dorsal root ganglia (page 295). They are acted upon by external stimuli, while their impulses in turn act on the neurons in the spinal cord.
[pg 310]2. Through the central system (spinal cord or base of brain). This, the intermediate division, may be composed of mon-axonic neurons, or it may consist of branches from the afferent neurons. In the case of separate neurons, these are acted upon by impulses from the afferent neurons, while their impulses serve in turn as stimuli to other neurons within the cord (Fig. 129).
3. From the central nervous system to the muscles. This, the efferent division, is made up of mon-axonic neurons. Most of these have their cell-bodies in the gray matter of the cord, while their fibers pass into the spinal nerves by the ventral roots.107 They may be stimulated by impulses either from the intermediate neurons, or from branches of the afferent neurons. Their impulses reach and stimulate the muscles.
Reflex Action in Digestion.—The flowing of the saliva, when food is present in the mouth, is an example of reflex action. In this case, however, the organ excited to activity is a gland instead of a muscle. The food starts the impulses, and these, acting through the bulb, reach and stimulate the salivary glands. In a similar manner food excites the glands that empty their fluids into the stomach and intestines, and stimulates the muscular coats of these organs to do their part in the digestive process. To a considerable extent, neurons having their cell-bodies in the sympathetic ganglia are concerned in these actions (Fig. 139).
Fig. 139—Diagram illustrating reflex action in its relation to the food canal. The nerve path in this case includes sympathetic neurons.
Reflex Action in the Circulation of the Blood.—On sudden exposure[pg 311] to cold, the small arteries going to the skin quickly diminish in size, check the flow of blood to the surface, and prevent too great a loss of heat. In this case, impulses starting at the surface of the body are transmitted to the bulb and then through the efferent neurons to the muscles in the walls of the arteries. In a somewhat similar manner, heat leads to a relaxation of the arterial walls and an increase in the blood supply to the skin. Other changes in the blood supply to different parts of the body are also of the nature of reflex actions. As in the work of digestion, neurons having their cell-bodies in the sympathetic ganglia aid in the control of the circulation.
Purposes of Reflex Action.—The examples of reflex action so far considered illustrate its two main purposes—(1) protection, and (2) a means of controlling important processes.
The pupil has but to study carefully the reflex actions of his own body for a period, say of two or three weeks, in order to be convinced of their protective value. He will observe that portions of his body have, on exposure to danger, been moved to places of safety, while in some instances, like falling, his entire body has been adjusted to new conditions. He will also find that reflex action is quicker, and for that reason offers in some cases better protection, than movements directed by the mind. In digestion and circulation are found the best examples of the control of important processes through reflex action.
Voluntary Action.—It is observed that reflex action, in the sense that it has so far been considered, is not the usual mode of action of the external organs, but is, instead, a kind of emergency action, due to unusual conditions and excitation by strong stimuli. Voluntary actions, on the other hand, represent the ordinary, or normal, action of these organs. They comprise the movements of the body of which we are conscious and which are controlled by the mind. But while they are of a higher order than reflex[pg 312] actions and are under intelligent direction, they are brought about in much the same manner.
Voluntary Action Pathways differ in but one essential respect from those of reflex action. They pass through the cerebrum, the organ of the mind (Fig. 140). This is necessary in order that the mind may control the action. From all portions of the body surface, afferent pathways may be traced to the cerebrum; and from the cerebrum efferent pathways extend to all the voluntary organs. A complex system of intermediate neurons, found mostly in the brain, join the afferent with the efferent pathways. The voluntary pathways are not distinct from, but include, reflex pathways, a fact which explains why the same external stimulus may excite both reflex and voluntary action (Fig. 141).
Fig. 140—Diagram of a voluntary action pathway.
Choice in Voluntary Action.—In reflex action a given stimulus, acting in a certain way; produces each time the same result. This is not the case with voluntary action, the difference being due to the mind. In these actions the external stimulus first excites the mind, and the resulting mental processes—perhaps as memory of previous experiences—supply a variety of facts, any of which may act as stimuli to action. Before the action takes place, however, [pg 313]some one fact must be singled out from among the mental processes excited. This fact becomes the exciting stimulus and leads to action. It follows, therefore, that the action which finally occurs is not necessarily the result of an immediate external stimulus, but of a selected stimulus—one which is the result of choice.
Fig. 141—Diagram of voluntary action pathways including reflex pathways.
Not only does the element of choice enter into the selection of the proper stimulus, but it also enters into the time, nature, and intensity of the action. For these reasons it is frequently impossible to trace voluntary actions back to their actual stimuli. The pupil will recognize the element of choice in such simple acts as picking up some object from the street, complying with a request, and purchasing some article from a store.
Reflex and Voluntary Action Compared.—Certain likenesses and differences, already suggested in these two forms of action, may now be more fully pointed out. Reflex and voluntary action are alike in that the primary cause of each is some outside force or condition which has impressed itself upon the nervous system. They are also alike in the general direction taken by the impulses in producing the action. The impulses are, first, from the surface of the body to the central nervous system; second,[pg 314] through the central system; and third, from the central nervous system to the active tissues of the body.
Their chief differences are to be found, first, in the pathways followed by the impulses, which are through the cerebrum (the organ of the mind) in voluntary action, but in reflex action are only through the spinal cord or the lower parts of the brain; and second, in the fact that voluntary action is under the direction of the mind, while reflex action is not. It would seem, therefore, that the statement sometimes made that "voluntary action is reflex action plus the mind" is not far from correct. Mind, however, is the important factor in this kind of action.
Secondary Reflex Action.—Everyday experience teaches that any voluntary action becomes easier by repetition. A given act performed a number of times under conscious direction establishes a condition in the nervous system that enables it to occur without that direction and very much as reflex actions occur. Actions of this kind are known as secondary reflex actions, or as acquired reflexes. Walking, writing, and numerous other movements pertaining to the occupation which one follows are examples of such reflexes. These activities are at first entirely voluntary, but by repetition they gradually become reflex, requiring only the stimulus to start them.
The advantages to the body of its acquired reflexes are quite apparent. The mind does not have to attend to the selection and direction of stimuli and, to that extent, is left free for other work. A good example of this is found in writing, where the mind apparently gives no heed to the movements of the hand and is only concerned in what is being written. The student will easily supply other illustrations of the advantages of secondary reflex action.
[pg 315]The development of secondary reflexes probably consists in the establishment of fixed pathways for impulses through the nervous system. Through the branching of the nerve fibers many pathways are open to the impulses. But in repeating the same kind of action the impulses are guided into particular paths, or channels. In time these paths become so well established that the impulses flow along them without conscious direction and it is then simply necessary that some stimulus starts the impulses. By following the established pathways, these reach the right destination and produce the desired result. According to this view, secondary reflex action is but a higher phase of ordinary reflex action—a kind of reflex action, the conditions of which have been established by the mind through repetition. (See functions of the cerebellum, page 317.)
Habits.—People are observed to act differently when exposed to the same conditions, or when acted upon by the same stimuli. This is explained by saying they have different habits. By habits are meant certain general modes of action that have been acquired by repetition. Certain acts repeated again and again have established conditions in the nervous system which enable definite forms of action to be excited, somewhat after the manner of reflex action. On account of habits, therefore, the actions of the individual are more or less predisposed. What he will do under certain conditions may be foretold from his habits. Habits simply represent, a higher order of secondary reflexes—those more closely associated with the mental life and character than are the lower forms.
Habits, in common with other forms of secondary reflex action, serve the important purpose of economizing the nervous energy. However, if pernicious habits are formed instead of those that are useful, they are detrimental from both a moral and physical standpoint. Youth is recognized as the period in which fundamental habits are formed and character is largely determined. Therefore parents[pg 316] and teachers do wisely when they insist upon the formation of right habits by the young.
Functions of Divisions of the Nervous System.—The relationship between the different parts of the nervous system is very close and one part does not work independently of other parts. At the same time the general work of the nervous system requires that its different divisions serve different purposes:
1. The peripheral divisions of the nervous system are concerned in the transmission of impulses between the surface of the body and the central system and between the central system and the active tissues. The nerves are the carriers of the impulses. The ganglia contain the cell-bodies which serve as nutritive centers; and, in the case of the sympathetic ganglia, these cell-bodies are the places where the fiber terminations of one neuron connect with, and stimulate, other neurons.
2. The gray matter in the spinal cord, bulb, pons, and midbrain (through the cell-bodies, fiber terminations, and short neurons which they contain) completes the reflex action pathways between the surface of the body and the voluntary muscles, and also between the surface of the body and the organs of circulation and digestion.
3. The white matter of the spinal cord, bulb, pons, and midbrain (by means of the fibers of which they are largely composed) forms connections with, and passes impulses between, the various parts of the central nervous system.
4. The bulb, because of certain special reflex-action pathways completed through it, is the portion of the central nervous system concerned in the control of respiration, circulation, and the secretion of liquids.
Work of the Sympathetic Ganglia and Nerves.—The neurons which form these ganglia aid in controlling the vital processes, especially digestion[pg 317] and circulation. These neurons are controlled for the most part by fibers from the bulb and spinal cord, and cannot for this reason be looked upon as forming an independent system. Their chief purpose seems to be that of spreading the influence of neurons from the central system over a wider area than they would otherwise reach. For example, a single neuron passing out from the spinal cord may, by terminating in a sympathetic ganglion, stimulate a large number of neurons, each of which will in turn stimulate the cells of muscles or of glands. Because of this function, the sympathetic neurons are sometimes called distributing neurons.
Functions of the Cerebellum.—Efforts to discover some special function of the cerebellum have been in the main unsuccessful. Its removal from animals, instead of producing definite results, usually interferes in a mild way with a number of activities. The most noticeable results are a general weakness of the muscles and an inability on the part of the animal to balance itself. This and other facts, including the manner of its connection with other parts of the nervous system, have led to the belief that the cerebellum is the chief organ for the reflex coördination of muscular movements, especially those having to do with the balancing of the body. In this connection it is subordinate to and under the control of the cerebrum. Of the relations which the cerebellum sustains to the cerebrum and to the different parts of the body, the following view is quite generally held:
In the development of secondary reflexes, as already described, conditions are established in the cerebellum, such that given stimuli may act reflexively through it and produce definite results in the way of muscular contraction. After the establishment of these conditions, afferent impulses from the eyes, ears, skin, and other places, under the general direction of the cerebrum, may cause such actions as the balancing of the body, walking, etc., as well as the delicate and varied movements of the hand. This view of its functions makes of the cerebellum the great center of secondary reflex action.
Functions of the Cerebrum.—While the work of the cerebrum is closely related to that of the general nervous system, it, more than any other part, exercises functions peculiar to itself. The cerebrum is the part of the nervous system upon which our varied experiences leave their impressions and through which these impressions are made[pg 318] to influence the movements of the body. But the power to alter, postpone, or entirely inhibit, nervous movements is but a part of the general work ascribed to the cerebrum as the organ of the mind. Numerous experiments performed upon the lower animals, together with observations on man, show the cerebrum to be the seat of the mental activities, and to make possible, in some way, the processes of consciousness, memory, volition, imagination, emotion, thought, and sensation.
Localization of Cerebral Functions.—Many experiments have been performed with a view to determining whether the entire cerebrum is concerned in each of its several activities or whether special functions belong to its different parts. These experiments have been made upon the lower animals and the results thus obtained compared with observations made upon injured and imperfectly developed brains in man. The results have led to the conclusion that certain forms of the work of the cerebrum are localized and that some of its parts are concerned in processes different from those of others.
Fig. 142—Location of cerebral functions. Diagram of cerebrum, showing most of the areas whose functions are known.
The work of locating the functions of different parts of the cerebrum forms one of the most interesting chapters in the history of brain physiology. The portions having to do with sight, voluntary motion, speech, and hearing have been rather accurately determined, while considerable evidence as to the location of other functions has been secured. Much of the cerebral surface, however, is still undetermined (Fig. 142).
NERVOUS CONTROL OF IMPORTANT PROCESSES
Circulation of the Blood.—1. Control of the Heart.—The ability to contract at regular intervals has been shown to reside in the heart [pg 319]muscle. Among other proofs is that furnished by cold-blooded animals, like the frog, whose heart remains active for quite a while after its removal from the body. These automatic contractions, however, are not sufficient to meet all the demands made upon the circulation. The needs of the tissues for the constituents of the blood vary with their activity, and it is therefore necessary to vary frequently the force and rapidity of the heart's contractions. Such changes the heart itself is unable to bring about.
For the purpose of controlling the rate and force of its contractions, the heart is connected with the central nervous system by two kinds of fibers:
a. Fibers that convey excitant impulses to the heart to quicken its movements.
b. Fibers that convey inhibitory impulses to the heart to retard its movements.
The cell-bodies of the excitant fibers are found in the sympathetic ganglia, but fibers from the bulb connect with and control them. The cell-bodies of the inhibitory fibers are located in the bulb, from where their fibers pass to the heart as a part of the vagus nerve.
In addition to the fibers above mentioned, are those that convey impulses from the heart to the bulb. These connect with neurons that in turn connect with blood vessels and with them act reflexively, when the heart is likely to be overstrained, to cause a dilation of the blood vessels. This lessens the pressure which the heart must exert to empty itself of blood. These fibers serve, in this way, as a kind of safety valve for the heart.
2. Control of Arteries.—Changes in the rate and force of the heart's contractions can be made to correspond only to the general needs of the body. When the blood supply to a particular organ is to be increased or diminished, this is accomplished through the muscular coat in the arteries. The connection of the arterial muscle with the sympathetic ganglia and the method by which they vary the flow of blood to different organs has already been explained (pages 311 and 49), so that only the location of the controlling neurons need be noted here. These, like the controlling neurons of the heart, have their cell-bodies in the bulb. It thus appears that the entire control of the circulation is effected in a reflex manner through the nerve centers in the bulb. These centers are stimulated by conditions that relate to the movement of the blood through the body.
[pg 320]Respiration.—Efferent fibers connect the different muscles of respiration with a cluster of cell-bodies in the bulb, called the respiratory center. This center together with the nerves and muscles in question form an automatic, or self-acting, mechanism similar in some respects to that of the heart. Through the impulses passing from the respiratory center to the muscles, a rhythmic action is maintained sufficient to satisfy the usual needs of the body for oxygen. The demand of the body for oxygen, however, varies with its activities, and to such variations the respiratory center alone is unable to respond. The regulating factor in the respiratory movements has been found to be the condition of the blood with reference to the presence of oxygen and carbon dioxide. If the blood contains much carbon dioxide and little oxygen, it acts as a strong stimulus to the respiratory center, causing it, in turn, to stimulate the respiratory muscles with greater intensity and frequency. On the other hand, if the blood contains much oxygen and little carbon dioxide, it acts only as a mild stimulus. This explains how physical exercise increases the breathing, since the muscles at work consume more oxygen than when resting and give more carbon dioxide and other wastes to the blood.
The respiratory center is also connected by afferent nerves with the mucous membrane of the air passages. Irritation of the nerve endings in this membrane causes impulses to pass to the center, and this leads, by reflex action, to such modifications of the respiratory acts as sneezing and coughing. There is also a connection between the cerebrum and the respiratory center. This is shown by the fact that one can voluntarily change the rate and force of the respiratory movements, and further by the fact that emotions affect the breathing.
Regulation of the Body Temperature.—As explained in the study of the skin (page 270), the nervous system regulates the body temperature by controlling the circulation of the blood through the skin and the internal organs. This is accomplished by stimulating in a reflex manner the muscles in the walls of certain arteries. To prevent the body from getting too hot, muscles in the arteries going to the skin relax, thereby allowing more blood to flow to the surface, where the heat can be disposed of through radiation and through the evaporation of the perspiration. On the other hand, if the body is in danger of losing too much heat, the muscles in the walls of arteries going to the skin are made to contract and those to internal organs relax, so that less blood flows to the skin and more to the internal organs. In this[pg 321] way the nervous system adjusts the circulation to suit the conditions of temperature outside of and within the body and, in so doing, maintains the normal body temperature.
Summary.—The nervous system is able to control, coördinate, and adjust the different organs of the body through its intimate connection with all parts and through a stimulus (the nervous impulse) which it supplies and transmits. Nervous impulses, excited by external stimuli, follow definite paths and cause activity in the different parts of the body. All such pathways are through the central nervous system. In reflex action the impulses are mainly through the spinal cord, but to some extent through the bulb, pons, and midbrain. In voluntary action they pass through the cerebrum—a condition that leads to important modifications in the results. The cerebrum, in addition to controlling the voluntary movements, is able to establish the necessary conditions for secondary reflex actions, such as walking, writing, etc. Although certain of the divisions of the nervous system exercise special functions, all parts of it are closely related.
Exercises.—1. Give the function of each of the parts of a neuron.
2. State the purpose of the nervous impulse.
3. Show that the exciting cause of bodily action is outside of the nervous system and, to a large extent, outside of the body.
4. Describe the arrangement that enables stimuli outside of the body to cause action within the body.
5. Describe a reflex action and show how it is brought about.
6. Distinguish between afferent, efferent, and intermediate neurons.
7. Draw diagrams showing the impulse pathways in voluntary and in reflex action.
8. What purposes are served by the sympathetic neurons?
9. Describe the method of control of the circulatory and digestive processes. How do reflex actions protect the body?
[pg 322]10. Compare voluntary and reflex action. In what sense are all the activities of the body reflex?
11. In what sense is walking voluntary? In what sense is it reflex?
12. How does secondary reflex action lessen the work of the nervous system?
13. State the special functions of the nerves, ganglia, spinal cord, bulb, cerebellum, and cerebrum.
14. State the importance of the formation of correct habits.
Fig. 143—Nerve board for demonstrating nerve pathways.
PRACTICAL WORK
To demonstrate Nerve Pathways.—A smooth board, 2x6 ft., is painted black, and upon this is drawn in white a life-size outline of the body. Pieces of cord of different colors and lengths are knotted to represent mon-axonic and di-axonic neurons. These are then pinned or tacked to the board in such a manner as to represent the connections in the different kinds of nerve pathways. Fig. 143 shows such a board with connections for a reflex action and a voluntary action of the same muscle.
Study of the "Knee Jerk" Reflex.—A boy is seated on a chair with the legs crossed. With a small pointer he is given a light, quick blow on the upper margin of the patella at the point of connection of the tendon. The stroke will usually be followed by a reflex movement of the foot. Does this take place independently of the mind? (The one upon whom the experiment is being performed should assume a relaxed condition and make no effort either to cause or prevent the movement.) Can the movement be[pg 323] inhibited (prevented)? Repeat the experiment, effort being made to prevent the movement, but not by contracting opposing muscles.
Other reflex actions adapted to class study are those of the eyes, such as the closing of the lids on moving objects near them and the dilating of the pupils when the eyes are shaded. The involuntary jerking of the head on bringing the prongs of a vibrating tuning fork in contact with the end of the nose is also a reflex action which can be studied to advantage.
To determine the Reaction Time.—Have several pupils join hands, facing outwards, making a complete circle, excepting one gap. Give a signal by touching the hand of one pupil at the end of the line. Let this pupil communicate the signal, by pressure of the other hand, to the next pupil and so on around, having the last pupil raise the free hand at close of the experiment. Note carefully the time, preferably with a stop watch, required to complete the experiment and divide this by the number of pupils, to get the average reaction time. The experiment may be repeated with boys only and then with girls, comparing their average reaction time.
Reflex Action of the Salivary Glands.—Place a small pinch of salt upon the tongue and note the flow of saliva into the mouth. Try other substances, as starch, bits of wood, and sugar. What appears to be the natural stimulus for these glands? Compare with reflex actions of the muscles.
CHAPTER XIX - HYGIENE OF THE NERVOUS SYSTEM
The far-reaching effects and serious nature of disorders of the nervous system are sufficient reasons for considering carefully those conditions that make or mar its efficiency. Controlling all the activities of the body and affecting through its own condition the welfare of all the organs, the hygiene of the nervous system is, in a large measure, the hygiene of the entire body. Moreover, it is known that some of our worst diseases, including paralysis and insanity, are disorders of the nervous system and are prevented in many instances by a proper mode of living.
The Main Problem.—Many of our nervous disorders are undoubtedly due to the age in which we live. Our modern civilization, with all its facilities for human advancement and enjoyment, throws an extra strain upon the nervous system. Educational and social standards are higher than ever before and life in all its phases is more complex. Since we can hardly change the conditions under which we live, and probably would not if we could, we must learn to adapt or adjust ourselves to them so as to secure for the nervous system such relief as it requires. This adjustment is sometimes difficult, even when the actual needs of the nervous system are known.
The healthful action of the nervous system requires, on the one hand, exercise, but on the other hand, a certain condition of quietude, or poise—a state which is directly opposed to that of restlessness. The conditions of modern[pg 325] life seem able to force upon the nervous system all the exercise that it needs, and more (whether it be of the right kind or not), so that the main problem of to-day seems to be that of conserving, or economizing, the nervous energy and of preventing nervous waste.
Wasteful Forms of Nervous Activity.—There are without doubt many forms of activity that waste the vital forces of the body and lead to nervous exhaustion. Take, for example, the rather common habit of worrying over the trivial things of life. Certainly the nervous energy spent in this way cannot be used in doing useful work, but must be counted as so much loss to the body. One who would use his nervous system to the best advantage must find some way of preventing waste of this kind.108
Undue excitement, as well as pleasurable dissipations, also tend toward nervous exhaustion. And while the fact is recognized that pleasurable activities supply a necessary mental exercise, the limit of healthful endurance must be watched and excesses of all kinds avoided. Intense emotional states are found to be exhausting in the extreme; and the suppression of such undesirable feelings as anger, fear, jealousy, and resentment are of immense value in the hygiene of the nervous system.
The Habit of Self-control.—Much of the needless waste of nervous energy, including that of worrying over trivial matters, may be prevented through the exercise of self-control. From the standpoint of the nervous system, the present age differs from the past mainly in supplying a[pg 326] greater number and variety of nerve stimuli. Self-control means the ability to suppress activities that would result from undesirable stimuli and to direct the bodily activities into channels that are profitable. Self-control, therefore, is not only to be exercised on occasions of great emergency, but in the everyday affairs of life as well. It is even more important that the daily toiler at his task be able to keep the petty annoyances of life from acting as irritants to his nervous system than that he keep cool during some great calamity. The habit of self-control is acquired mainly through the persistent effort to prevent any and all kinds of petty annoyances from affecting the nerves or the temper.
Nervousness.—Self-control is much more easily practiced by some than by others. This is due partly to habit, but is also due to an actual difference in the degree of sensitiveness, or irritability, of the nervous systems of different people. One whose nervous system tends to respond too readily to any and all kinds of stimuli is said to be "nervous." This condition is in some instances inherited, but is in most cases due to the wasteful expenditure of nervous energy or to the action of some drug upon the body. Excess of mental work, too much reading, long-continued anxiety, eye strain, and the use of tea, coffee, alcohol, tobacco, or other drugs, including many of those taken as medicines, are known to cause nervousness. Nervousness is not only a source of great annoyance, both to one's self and to others, but is a menace to the general health.
The first step toward securing relief from such a condition is the removal of the cause. The habits should be inquired into and excesses of all kinds discontinued. In some instances it may be necessary to have the eyes [pg 327]examined and glasses fitted by a competent oculist.109 The nervous energy should be carefully economized and the habit of self-control diligently cultivated. Special exercises that have for their purpose the equalizing of the circulation and the strengthening of the blood vessels of the neck and the brain also have beneficial effects.
Nervous Overstrain.—Both mental and physical overwork tends to weaken the nervous system and to produce nervousness. Where hard mental work is long continued, or where it is carried on under excitement, a tense nervous condition is developed which is decidedly weakening in its effects. The causes which lead to such a condition, and in fact overwork of all kinds, should if possible be avoided. Where this is not possible, and in many cases it is not, the period of overwork should be followed by one of rest, recreation, and plenty of sleep. To the overworked in body or in mind, nothing is more important from a hygienic, as well as moral, standpoint, than the right use of the one rest day in seven. The best interests of our modern civilization require that the Sabbath be kept as a quiet, rest-giving day.
Disturbed Circulation of the Brain.—Nervousness not infrequently is accompanied by an increase in the circulation of the brain and disappears when this condition is relieved. Though mental work and excitement tend naturally to increase the circulation in the brain, this should subside with rest and relief from excitement. When there is a tendency[pg 328] for this condition to become permanent, effort should be made looking for relief. Increasing the circulation in the lower extremities by hot or cold foot baths, or by much walking, is found to be most beneficial. Special exercises of the muscles of the neck are also recommended as a means of relieving this condition.110
Hygienic Value of Work.—Within reasonable limits, both mental and physical work are conducive to the vigor of the nervous system. Through work the energies of the body find their natural outlet, and this prevents dissipation and the formation of bad habits. Even hard work does not injure the nervous system, and severe mental exertion may be undergone, provided the proper hygienic conditions are observed. The nervous disorders suffered by brain workers are not, as a rule, due to the work which the brain does, but to violation of the laws of health, especially the law of exercise. Such persons should observe the general laws of hygiene and especially should they practice daily those forms of physical exercise that tend to counteract the effects of mental work.
Physical Exercise properly taken is beneficial to the nervous system through both direct and indirect effects. A large proportion of the nerve cells have for their function the production of motion, and these are called into play only through muscular activity. Then, as already suggested, physical exercise counteracts the unpleasant effects of mental work. Hard study causes an excess of blood to be sent to the brain and a diminished amount[pg 329] to the arms and to the legs. Physical exercise redistributes the blood and equalizes the circulation. Light exercise should, therefore, follow hard study. The student before retiring at night is greatly aided in getting to sleep and is put in a better condition for the next day's work by ten to fifteen minutes of light gymnastics. A daily walk of two or three miles is also an excellent means of counteracting the effects of mental work. The brain worker should, however, avoid violent exercise or the carrying of any kind of exercise to exhaustion.
Sleep, and plenty of it, is one of the first requirements of the nervous system. It is during sleep that the exhausted brain cells are replenished. To shorten the time for sleep is to weaken the brain and to lessen its working force. No one should attempt to get along with less than eight hours of sleep each day and most people require more. Children require more sleep than adults. Those under six years should have from eleven to twelve hours of sleep per day. Children between six and ten years should have at least ten hours.
Insomnia, or sleeplessness, on account of its effects upon the nervous system, is to be regarded as a serious condition, and hygienic means for relieving it should be diligently sought. Having its cause in nervousness, a disturbed circulation of the brain, or some form of nervous exhaustion, it is benefited through relieving these conditions and in the manner already described. Of course the external conditions for aiding sleep should not be overlooked. The bed should be comfortable, and the room should be cool, well ventilated, dark, and quiet. The inducing of sleep by means of drugs is a dangerous practice and should never be resorted to except under the direction of the physician.
[pg 330]Effects of Heat and Cold.—Heat and cold both have their effects upon the nervous system. Heat increases the nervous irritability, while cold acts as a natural sedative to the nerves. A nervous person is made more nervous by an overheated atmosphere, but derives beneficial effects from exposing the body freely to cold air and water. The tonic cold bath (page 273), if taken with the usual precautions, can be used to good advantage in diminishing nervousness. The taking of outdoor exercise in cold weather is, for the same reason, an excellent practice.
Effect of Emotional States.—We have already noted the effect of certain emotional states upon the digestion of the food (page 162). Emotional states are also known to interfere with breathing and with the action of the heart. Such effects are explained through the close relation of the mind to the work of the nervous system in general. While certain emotional states, such as fear, anger, melancholia, and the impulse to worry, interfere seriously with the normal action of the nervous system, others, such as contentment, cheerfulness, and joy, are decidedly beneficial in their effects. How important, then, is the habit of suppressing the states that are harmful and of cultivating those that are beneficial. From a hygienic, as well as social, standpoint a cheerful, happy disposition is worth all the effort necessary for its attainment.
The Nervous Condition of Children should be a matter of deep concern on the part of both parents and teachers. In the home, as well as in the school, the child may be "pushed" until the nervous system receives permanent injury. Exhaustion of nerve cells is produced through too many and too vivid impressions being made upon the immature brain. The child should be protected from[pg 331] undue excitement. He should have the benefit of outdoor exercise and should be early inured to cold. He should be shielded from the poisoning effects of tea, coffee, tobacco, alcohol, and other drugs. He should have impressed upon him the habit of self-control. He should not be indulged in foolish caprices or whims, but should be taught to be content with plain, wholesome food and with the simple forms of enjoyment.
Influences at School.—School life is necessarily a great strain upon the child. Night study added to the work of the day makes a heavy burden for elementary pupils to bear. Though the legal school age is usually fixed at six years, delicate children should be kept out of school until they are seven or eight years old, provided they have good homes. In addition to the excitation incident to studying and reciting lessons, conditions frequently arise both in the schoolroom and upon the playground that create a feeling of fear or dread in the minds of children. Quarrels and feuds among the children and the bullying of big boys on the playground may work untold harm. All conditions tending to develop fear, uneasiness, or undue excitement on the part of children should receive the attention of those in authority.
Excessive Reading is a frequent cause of injury to the nervous systems of children. This has a bad effect, both on account of too many impressions being made upon the mind and also on account of the strain to the eyes. Then if the reading consists mostly of light fiction, the mind is directed away from the really important things of life. The reading of children should be thoughtfully controlled, both as to quality and quantity. Exciting stories should, as a rule, be excluded, but a taste for biography, historical and scientific writings, and for the great works[pg 332] of literature should be cultivated. Simple fairy tales which have a recognized value in developing the imagination of the child need not be omitted, but it is of vital importance that the "story-reading habit" be not formed.
Effects of Drugs.—Because of its delicacy of structure a number of chemical compounds, or drugs, are able to produce injurious effects upon the nervous system. Some of these are violent poisons, while others, in small quantities, are mild in their action. Certain drugs, in addition to their immediate effects, bring about changes in the nervous system which cause an unnatural appetite, or craving, that leads to their continued use. This is the case with alcohol, the intoxicating substance in the usual saloon drinks, and with nicotine, the stimulating drug in tobacco. The same is also true of morphine, chloral, and several other drugs used as medicines. The danger of becoming a slave to some useless and pernicious habit should dissuade one from the use of drugs except in cases of positive emergency.
Alcohol and the Nervous System.—Alcohol, as already shown, injures practically all portions of the body; but it has its worst effects upon the nervous system. Through its action on this system, it interferes with the circulation of the blood, produces a condition of "temporary insanity" called intoxication, weakens the will, and eventually dethrones the reason. Worst of all, it produces a condition of "chronic poisoning" which manifests itself in an unnatural craving, and this causes it to be used by the victim even when he knows he is "drinking to his own destruction." Though its use in small quantities does not, as a rule, produce such marked effects upon the nervous system, it develops the "craving," and this is apt in time to lead to its use in larger quantities. But even if this does not occur, the practice is objectionable for its unhygienic effects[pg 333] in general.111 Tippling with such mild solutions of alcohol as light wine, beer, and hard cider is, for these reasons, a dangerous pastime.
Alcohol and Crime.—It is sometimes stated that no one who leaves alcohol alone will be injured by it. This is true only of its direct effects; not of its indirect effects. Whenever a crime is committed somebody is injured, and alcohol is known to be a chief cause of crime. Alcohol causes crime through the loss of self-control, seen especially in intoxication, and also because of the moroseness and quarrelsomeness which it developes in certain individuals. Indirectly it causes crime through the poverty which it engenders and through its influence in bringing about social conditions out of which crime develops. Everything considered, the free use of alcohol is incompatible with the nervous health and moral tone of a community.
Nicotine and the Nervous System.—Nicotine is an oily substance which is extracted from the tobacco plant. Its action on the nervous system is in general that of a poison. Taken in small quantities, it is a mild stimulant and, if the doses are repeated, a habit is formed which is difficult to break. Tobacco is used mainly for the stimulating effect of this drug. While not so serious in its results as the alcohol and other drug habits, the use of tobacco is of no benefit, is a continual and useless expense, and, in many instances, causes a derangement of the healthy action of the body.112[pg 334] With the bad effects of the nicotine must be included those of questionable substances added to the tobacco by the manufacturer, either for their agreeable flavor or for adulteration.
Relation of Age to the Effects of Nicotine.—The use of tobacco by the young is especially to be deplored. In addition to the harmful effects observed in those of mature years, nicotine interferes with the normal development of the body and lays, in many instances, the foundation for physical and mental weakness in later life. The cigarette is decidedly harmful, especially when inhalation is practiced, its deadening effects being in part due to the wrappers, some of which have been shown to contain arsenic and other poisonous drugs. While dulling the intellect and weakening the body, cigarette smoking also tends to make criminals of boys.113 Parents, teachers, school officers, and all who have the good of mankind at heart should take every precaution, including that of setting a good example, to prevent the formation of the tobacco habit by those of immature years.
Habit versus Self-control.—The power of self-control, already emphasized for its importance in the economical expenditure of the nervous energy, is of vital importance in its relation to the habits of the body. Self-control is the chief safeguard against the formation of bad habits and is the only means of redemption from such habits after they have once been formed. The persistent cultivation of the power to control the appetites and the passions, as well as all forms of activity which tend to injure the body or debase the character, gives a tone to the nervous system[pg 335] which increases the self-respect and raises the individual to a higher plane of life. The worst habits can be broken and good ones formed in their stead, if only there is sufficient determination to accomplish these results. Failure comes from not having the mind thoroughly "made up" and from not having, back of the desire to do better, "the strong will of a righteous determination."
Effects of External Conditions.—While the inner life and habits have most to do with the hygiene of the nervous system, a certain amount of attention may properly be given to those conditions outside of the body which affect directly or indirectly the state of this system. Noise, disorder, and confusion act as nervous irritants, but quiet, order, and system have the opposite effect. There is, therefore, much in the management of the office, factory, schoolroom, or home that has to do with the real hygiene of the nerves as well as with the efficiency of the work that is being done. The suppression of distracting influences not only enables the mind to be given fully to the work in hand, but actually prevents waste of nervous energy. Although the responsibility for securing the best conditions for work rests primarily with those in charge, it is also true that each individual in every organization may contribute to the order or disorder that prevails.
Social Relations.—In considering the external conditions that affect the nervous system, the fact must not be overlooked that man is a social being and has to adjust himself to an established social order. His relations to his fellow-men, therefore, affect strongly his nervous condition and theirs also. For this reason the best hygiene of the nervous system is based upon moral as well as physical right living. Along with the power of self-control and the maintenance of a correct nervous poise, there should be a proper regard[pg 336] for the welfare of others. On account of the ease with which one individual may disturb the nervous state of another, those social forms and customs which tend to establish harmonious relations among men are truly hygienic in their effects, and may well be carried out in spirit as well as "in letter."
It is also a fact that a given mental state in one person tends to excite a like state in those with whom he associates. How important, then, that each and all cultivate, as habits, the qualities of cheerfulness, kindness, and good-will, instead of the opposite states of mind. Especially in the family, and other groups of closely associated 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 noble, 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[pg 337] 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 quantities 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.