The science of life : fully illustrated in tone and line and including many diagrams

BOOK 1

a nerve is simply an enormous number of nerve-fibres lying side by side like the separate wires in a telephone cable, and bound together by a little connective tissue. As the nerve runs away from its root it branches, and the fibres begin to part company and disperse to their various destinations. Some of them are connected up to sense-organs, and carry to the central

Muscle

Fig. 39. The calf-muscle of a frog with its nerve.

If the nerve is pinched or otherwise stimulated the muscle contracts at once.

organization the information these organs obtain ; some go to the voluntary muscles and bear the impulses which make them contract ; some run to glands and make them secrete ; some plunge into the chest and belly and supply the more independent organs there, bringing impulses which slow them or hurry them up.

It is difficult to realize the full complexity of the nervous system. By dissecting as

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THE SCIENCE OF LIFE

CHAPTER 3

completely as possible the finer nerves in a limb, for example, one can get a fair idea of it. We will try in another way—by considering the control of the muscles as one only of the many sides of its activity.

The muscles that cause the wilful movements of man or mouse work under the direct impulsion of the central nervous system. To every muscle-fibre in a wellcontrolled muscle goes a nerve-fibre from a nerve-cell in that system and spreads out at last in a palm-like cluster of branches against its sides. Every movement made by that muscle-fibre is due to an impulse coming from the associated cell in the brain or spinal cord. Every movement made by a muscle depends upon a nerve made up of innumerable fibres. But there is hardly a movement of the body that is not the outcome of the co-ordinated action of a number of muscles, and hardly a moment of our waking life when we are not in movement. As the reader peruses this section he is performing continual slight movements of the muscles of his eyeballs and head and neck, so that his line of sight may run smoothly over the print. Now and again his arm moves up and his fingers turn the page. Perhaps at the same time he smokes and knocks the ash from a cigarette ; his lips may flicker in a smile of approval, or curl in contempt, or stretch spasmodically in a yawn. Here are muscles working by the score. And all the time the ninety-odd muscles that move his ribs continue their rhythmical contraction and _ relaxation. Every one of these movements involves the co-operating action of some thousands of muscle-fibres, and every one is precisely controlled by nervous impulses sent by his brain or spinal cord.

But the activities of the central nervous system extend further than this; they include a supervision of resting muscles. When a muscle is passive it is not allowed to hang limply from its attachments, but is kept braced and ready for immediate action when called upon. In the language of the physiologist it is in a state of slight partial contraction or tone. This tone is due to impulses from the central nervous system, and if its nerve is cut the muscle loses its tautness and becomes slack and flabby. Further, besides ‘keeping the muscles keyed up, the central nervous system seems to exert an influence on their general health, for cutting the nerve causes a slow shrinking of the muscle, which in the course of months or years may even disappear and give place to a mass of connective tissue. Perhaps this