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

WEARING OUT OF MACHINE AND ITS REPRODUCTION

As the brain expands there is a similar growth of the legs, accompanied by changes eccurring elsewhere in the body, and associated with the assumption of the upright posture. The most important of these changes is the development of the nervetracts leading from the brain to the limbmuscles, and enabling the latter to be accurately controlled by the former. At the same time the legs grow longer and stronger, and the lumbar region of the vertebral columnthe loms—elongates and consolidates to support the weight of the upper parts. “Learning to walk” is not a process of real learning at all; it follows gradually but automatically with the perfecting of these structures as a reflex follows automatically upon the construction of the senseorgan, nerve-connections, and muscles laid down before birth.

During the first two years or so there is another important process—the eruption or appearance of the milk teeth, the first of the two sets with which we are provided. The second set begins to replace the. first at about the fifth year, and it is not complete until the wisdom teeth have erupted in about the twenty-second year.

These are only the most obvious changes that occur during childhood. Throughout the body,in everyshred of tissue, development is going on. Moreover, this development continues until the parts actually wear out, for there is no wholly stationary period in the human life-cycle ; the body is always either developing or decaying.

It is not possible to name a definite prime, or climax, as it were, to human life, because our different tissues perfect themselves and wear out at different rates. The elastic and muscular coats of our arteries are at heir best at about twenty-five; this is when the professional athlete, who has to make sudden physical spurts, reaches his nighest point. The brain has a longer life ; it attains its most accurate control of the muscles between thirty and forty, and as an organ of thought it is probably at its dest between forty and fifty. In this there is a striking disharmony, for the circulatory system may stiffen and fail before the mental powers begin to fall off. The professional athlete has to retire and be forgotten long before his death, but the professional thinker generally has the privilege of dying in harness.

Perhaps the earliest sign of decay in the human body is the appearance of lines on the face—proof that the skin is already losing its elasticity.

Decay, then, is soon upon us; and this brings us again to the need for the revolution that happens in the early teens, the acquisition of the ability to reproduce, and the beginning of the insistent clamour that our reproductive organs set up so that we may be replaced.

In describing the ovary and testis we noted that in both glands, besides the tissue directly concerned with the manufacture of gametes, there is tissue of another kind; there are the interstitial cells. These interstitial cells, like the corpus luteum, are organs of internal secretion. They begin to work very early, long before any germ-cells are developed, and their secretions, carried round the body by the blood, determine most of the differences that distinguish man from woman.

The reproductive organs of thé male and the female are built upon a common type. To take an obvious example, the male has rudimentary nipples although he will never suckle his young. It is possible to go through the whole system, part by part, and show how male and female correspond. Now the very early embryo has a generalized reproductive system, one that is neither discernibly male nor female, but containing rudiments capable of developing into either the male or female form. The first sign of definite sexuality manifests itself in the testis or ovary, as the case may be, and it is the appearance of working interstitial tissue that sheds either male or female internal secretions into the blood. These substances affect the body and, by enlarging some of the parts and inhibiting the growth of others, make them develop into the characteristic type of one sex or the other. The female pelvis is broader than the male, so that it may support the pregnant uterus and allow the head of the baby to pass out. As early as the fifth month of embryonic development it is possible to make out a distinction in shape between the male and female pelvis, and this is also due to internal secretions from the interstitial cells.

After these early changes there is a long period of comparative rest throughout childhood. Sexual differentiation, although it is continuing, does not proceed as violently as it did before. But at puberty—at about fifteen years in the male and fourteen years in the female in temperate climates, and about two years earlier nearer the equator —the gametes begin to ripen, and with this acquisition of the reproductive faculty there is a violent burst of activity on the part of the interstitial cells. At this time the final distinctions between the sexes begin to be

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