Physiology and Hygiene for Secondary Schools Part 37

*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, 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]

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, 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 repet.i.tion. 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 repet.i.tion 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 ill.u.s.trations of the advantages of secondary reflex action.

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 repet.i.tion. (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 repet.i.tion. 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 a.s.sociated 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 and teachers do wisely when they insist upon the formation of right habits by the young.

*Functions of Divisions of the Nervous System.*-The relations.h.i.+p 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 pa.s.ses 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 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 pa.s.sing 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_ coordination 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 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]

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 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 const.i.tuents 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 r.e.t.a.r.d 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 pa.s.s 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.

*Respiration.*-Efferent fibers connect the different muscles of respiration with a cl.u.s.ter 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 pa.s.sing 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 pa.s.sages. Irritation of the nerve endings in this membrane causes impulses to pa.s.s 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 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, coordinate, 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 pa.s.s 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?