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

THE HARMONY AND DIRECTION OF THE BODY-MACHINE

that holds the body together for unified action, as the apparatus through which the whole body, with the billions of celldenizens and the endless variety of substances and secretions we have reviewed, is made one collective thing with a sense of itself and a purpose of its own.

We have already made use of a well-known comparison—that of the nervous system with a telephone system—and it is important at the outset to see how far the analogy is true and how far it is misleading. What in the nervous system corresponds to the telephone wire? What exactly are the channels along which part communicates with part ? To answer that question we must look into certain details of microscopic structure.

The cells of the nervous system differ from other cells in an important respect—their surfaces project into long, slender fibres of living substance. Fig. 38 represents a nerve-cell of the spinal cord. It has an irregular, star-shaped body, which is unusually large for a cell, and radiating away from this centre are a number of delicate protoplasmic arms (nerve-fibres) that spread and branch in the spinal cord. One of these arms, in this particular cell, is longer than the rest; it leaves the spinal cord and runs along a nerve to some organ in a distant part of the body. We shall follow it in a moment. The soft, pinkish tissue of the central nervous system consists entirely of such nerve-cells and their fibres, except for a little connective tissue to support them and bind them together, and these hundreds of millions of cells are the units—the clerks, so to speak, of the organization. The fibres correspond to the telephone wires in our analogy.

of finger-like branches which twine around and clasp the fibres of another. Others leave the central nervous system altogether and run to other tissuesto muscles, glands, sense-organs—and along these commands are sent out which control the working tissues, and reports from the sense-organs are carried in.

The telephone wires of the body, then, are

living threads of astounding delicacy, finer than the finest gossamer. They are about one-tenth the thickness of a human hair and may be several feet long. The messages which flash along these protoplasmic wires are called “nervous impulses’? ; they are

: Fig. 38. A single nerve-cell, much magnified.

Some of them run from nerve-cell Above is the cell-body, placed in the brain or spinal cord. Below, to nerve-cell—growing out from jhe nerve-fibre leads away and-supplies two muscle-cells. If the muscle one and ending in a tiny cluster supplied were in the foot the nerve-fibre, drawn to scale, would be a

quarter of a mile long.

physical changes which travel at about four hundred feet per second in man. Our bodies are permeated by a network of these fibres, centring in the brain and spinal cord.

Such of the fibres as go out from the brain and spinal cord to the rest of the body are collected together into bundles, the nerves ;