A Correct posture.

B = Incorrect posture.

is largely a matter of muscular training. In standing (Fig. 48), the head and body should be erect, the heels brought close together, and the shoulders brought into such a position that the back is approximately flat. In sitting (Fig. 49), care should be taken not to bend the body over the desk, and a proper relation between height of chair and desk should be secured.

Permanent curvature of the spine frequently results from carrying loads of books or other heavy objects on one side of the body only; pupils should therefore train themselves to use the arms alternately for this purpose.

IV. THE NERVOUS SYSTEM

228. The body as a collection of organs. In the preceding chapters we have discussed the digestive, respiratory, and circulatory systems and have seen that these organs furnish all parts of the body with food and oxygen. We have studied the process of oxidation whereby we keep warm and gain the power to do work. And finally we are familiar with the fact that the bones and muscles are the organs that give support to the body and provide the machinery Thus we see that the body is composed of many organs, each with its special function or functions.

Desk and seat too low. FIG. 49.-Sitting positions.

for all our motions.

But a human being is more

229. Coöperation of the organs. than a mere collection of working organs, for all the various organs work together for the common good. This is what we mean by coöperation (Latin, co = together operari to work). Suppose we take a few instances from everyday experience to illustrate this coöperation.

=

When food is taken into the mouth, the salivary glands pour out upon it an abundant supply of saliva. Now, the food never comes in contact with the glands. How is it, then, that they send out their secretion at just the right time and in the proper amount?

If a blow is aimed at one's face, one's hands immediately fly up to ward off the threatened injury. If the attack were pressed and one were really compelled to defend himself, his heart would beat much more rapidly, he would breathe faster, and the flow of perspiration would become evident. During the contest certain feelings, also, would doubtless be aroused.

230. Functions of the nervous system. - All the succession of activities just described would be utterly impossible if some means were not provided for making the organs work together for the common good. The arms could not see to strike at the antagonist; nor could the heart, lungs, or skin respond to the sudden exertion of the rest of the body. It is the nervous system that controls the action of each of the organs in the body and brings about a coöperation between them. All our sensations, too, and our will power are doubtless correlated with the activities of the nervous system.

The nervous system con

231. Parts of the nervous system. sists of nerve centers and nerve fibers (of which nerves are composed). The principal nerve centers are the brain and spinal cord (Fig. 50). These delicate organs are inclosed and wonderfully protected by the bony cranium and spinal column.

From the brain and spinal cord pass off numerous bundles of nerves. As they approach the different organs of the body they divide into branches, and thus the nerves become smaller and smaller. Finally, the microscope is needed to trace the individual nerve fibers to their endings in muscle, gland, or sense organ. By

means of these countless nerve fibers all parts of the body are put in communication with the nerve centers (see Fig. 50).

If a section is

232. Cellular structure of the nervous system. made of any part of the brain or spinal cord, two kinds of material, known respectively as gray matter and white matter, may be distinguished. In the gray matter are countless nerve cells (Fig. 51) which are very irregular in form. From most of the nerve cells project numerous fine processes that look like tiny branching roots. These bring the various nerve

cells into communication with each other.

One fiber-like process, however, has fewer branches than the others, and may be traced for a considerable distance from the cell body. This is the beginning of a nerve fiber, and it is the mass of nerve fibers that make up most of the white matter of the nervous system.

--nerve fiber

233. Nerve impulses. - We may compare nerve fibers to telegraph wires, and nerve impulses may be described as messages that pass along these fibers. But in making these comparisons we must remember that telegraphy and the action of the nervous system have, in all probability, little real resemblance. We know that nerves transmit impulses at the rate of about one hundred feet per second; electricity travels thousands of miles per second. Hence a nerve impulse cannot very closely resemble what we call a telegraph message. On the other hand, this nerve impulse travels much too rapidly to be explained as a chemical or mechanical action. We must therefore admit our ignorance of the real nature of the nervous impulse; nor do we know the real nature of the changes that take place in the nerve cells after receiv

FIG. 51.-Nerve cell from spinal cord.

ing the so-called message. The principal functions of the brain may for convenience be divided into (1) reflex activities, (2) conscious activities, and (3) automatic activities or habits.

234. Reflex activities. To illustrate the reflex action of the brain suppose we inhale some pepper; a message goes up the nerves to the cells in the nerve centers. This message is then reflected or switched off to cells which send impulses down the nerves that control the muscles of the chest. We then sneeze, and thus get rid of the pepper. Coughing, winking, blushing, the flow of saliva at the sight of savory food, — these are but a few of the reflex activites carried on by the brain.

235. Conscious activities. As long as we keep awake, countless nerve impulses keep pouring into our brains. When the cells of the gray matter receive these impressions, we usually become conscious that we are seeing, smelling, hearing, tasting, or feeling. These sensations are more or less lasting, too, for we can recall distinctly the appearance of objects that we saw yesterday, or even years ago, and we can hear again, as it were, the sounds we have heard in the past. In some unknown way these impressions are stored away in the protoplasm of the brain, and constitute our memory.

Another power of which we are conscious is the ability to direct the movements of the body. We can rise from a seat, walk about, talk or change the expression of our faces as we will.

236. Habitual activities. If we can remember the time when we learned to write, we recall that each letter was traced laboriously by a conscious effort of our brains to guide the muscles of our fingers. Writing, in our early years, belonged to the group of our conscious activities. But as time went on, less and less of our attention was needed for this mechanical process, until now our fingers seem to move of themselves. Walking, bicycle riding, swimming, playing the piano, conveying the food to our mouths - none of these activities require our attention. We have made these movements so many times that they have become automatic. In other words, the conscious part of our brains has trained other nerve centers to