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

BOOK 1

this end by local surgery, which for a time at least reanimates the declining tissue. Voronoff grafts new tissues, taken from young apes, in the effective region. There are other fairly obvious applications, by injection and so forth, of the same idea. Resort is also made to the substance and secretions of the thyroid, the pituitary body and other ductless glands which influence the growth processes. Steinach, Voronoff, and various other workers indubitably produced some remarkable rejuvenescences. But also they have had distinct failures and disappointments. Much more experiment is needed in this direction before we can confidently and safely hold back the advance of age.

In age, we are dealing not with an orderly decline, but with an irregular development and progressive exaggeration of what at first were minor maladjustments. The body is like a machine in which parts wear loose and little rifts widen. It was never perfectly adjusted and the defects appear in use. Each one of us ages after his nature and in his own fashion. A very capable, observant, and devoted medical attendant might by continual alertness check this strain, repair or replace that fagging part, and eke out a life far beyond the normal span. In some cases that might be worth while. And a general prolongation of vigour may be possible with more knowledge and care and improved surroundings. A time may come when men as a race will live more sanely and fully and suffer less. But it seems inconceivable that any individual human body will ever evade its final goal of death.

Nature’s way with life is to economize energy by setting a limit to recuperative processes. She prefers what is apparently the easier method of scrapping used individuals in favour of fresh ones, for whose appearance she has made the most generous and elaborate provision. She is very much like the automobile trade which prefers to sell new cars rather than keep on patching up the old, as they wear more and more out of easy working.

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Two sperms very highly

Fig. 58.

magnified seen From different angles to show the shape of the head.

THE SCIENCE OF LIFE

CHAPTER 4

§ 2 Reproduction and Fertilization

Among the millions of co-operating cells that build up a body a certain proportion is charged with the task of initiating new lives. They play no direct part in the chemical and energy changes of the community to which they belong. They are nourished, cleansed and warmed by the labours of the other tissues, but they give nothing in return, for they are the servants, not of the individual, but of the race.

The essential reproductive elements are minute; they are the “marrying cells” or gametes. They have the power of proliferating and differentiating in suitable circumstances and so giving rise to the tissues of a new body. There are two kinds of gametes—ova, or eggs, and spermatozoa, or sperms, and a new life can only be created by the union of one gamete of each kind, an ovum with a spermatozoon.

In this fact lies the reason for sexual differentiation. The human species is made up of two kinds of individuals, male and female, each having only one kind of gamete. The female possesses ova, the small globular cells that are capable of growing and developing into new bodies ; but before an ovum can do this it must be fertilized—that is to say, it must unite with a spermatozoon. The male possesses spermatozoa, and his part in the reproductive process is the providing of the spermatozoa that fertilize the ova.

The fertilized ovum of all the mammalia grows and develops inside the female and therefore her body contains, in addition to an ovum-factory, a special chamber where

Fig. 59. An ovum and a sperm drawn to the same scale (magnified about 200 diameters).

the embryo is tended and nourished. ‘There is no such special chamber in the bird, which lays its fertilized eggs. Nor is it usual in reptiles, fishes, and the majority of lower animals.