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

WEARING OUT OF MACHINE AND ITS REPRODUCTION

distended. By the end of pregnancy its weight, exclusive of the embryo and the embryonic membranes, is about three hundred times as great as normal. ‘This swelling up causes certain alterations in the disposition of the abdominal organs and may result in disturbances of digestion or the circulation. Besides the changes in the uterus, and the disturbances indirectly produced, pregnancy involves special modification in other organs; there is, for example, the preparation for suckling.

None of these adjustments are due to the action of the nervous system. ‘The nervous system is primarily concerned with rapid responses to one’s surroundings, and _ its part in reproduction is practically confined to the very elementary series of reflexes that bring the sexes together, and to certain other reflexes that occur during childbirth. The slower processes to which we have just referred are produced in another way ; they are regulated by internal secretion.

We have already shown how the activity of our organs can be modified as a result of alteration in chemical composition of the blood that bathes them, and we saw that there are special bodies, such as the adrenal gland, which shed substances into the bloodstream for the express purpose of affecting and controlling other tissues. In the regulation of the sexual functions similar processes play the leading part. Our knowledge of this regulation is developing rapidly at the time of writing, and like all rapidly developing subjects, it is still a little chaotic. There are brilliant lights and alluring obscurities. It is clear that the monthly rhythm is due to internal secretions produced in the ovary, which control the condition of the uterus, the breast, and so on; moreover, it is at least possible that the rhythm originates primarily not in the ovary at all, but in an anatomically unrelated structure lying at the base of the brain, the pituitary gland. This gland apparently produces an internal secretion that varies in amount with a monthly rhythm, and this secretion seems to affect the ovaries, modifying their activity, which, in turn, affects the uterus and breast.

It may be that the mechanism of the sexual rhythm is even more complicated than this. The thyroid gland, for example, which is another organ of internal secretion, often swells slightly during these phases and this may indicate active participation in the cycle. But these are undecided questions. We have said enough to show that the normal sexual rhythm is a very

complex thing, depending on minute but regular changes in the composition of the blood. Let us turn to the suspension of this rhythm that follows «fertilization, for in the phenomena of pregnancy the part played by internal secretion can be traced more clearly. Every time an ovum is shed from the ovary a special and temporary ductless gland is developed in that organ to see the ovum through.

We noted that in the ovary the ovum is tended and nourished by an embracing capsule of cells, the follicle. When the ovum escapes the follicle cells are relieved of this duty, but they do not lose their devotion to their charge; they multiply and alter somewhat in shape, forming a mass that fills the follicle and is called from its colour the “yellow body” (corpus luteum); and prepare to serve the ovum in another way. If the ovum is not fertilized the mass soon breaks down and disappears, but if fertilization occurs it remains and acts as an organ of internal secretion. It manufactures and sheds into the blood substances that are carried all round the body by the circulation and work various changes. The substances stimulate the inner layer of the uterus and prepare it for the reception and nourishment of the embryo ; they stimulate the secretion of milk; they soften the junction between the two halves of the hip girdle so that birth may be easy.

The corpus luteum is not the only organ that manifests special activity during pregnancy. It is probable that internal secretions are produced from other parts of the ovaryperhaps even from the placenta—which assist that of the corpus luteum in making the necessary modifications in the female organism. But the secretion of the corpus luteum is the most important.

One of the most interesting points about the internal secretions produced during pregnancy is that, like the hormones controlling sex-characters, they can directly influence the working of the brain. Shortly before birth occurs, a bitch prepares a special bed for her whelps and a doe rabbit plucks fur from her own breast to provide a soft, warm nest for her young. It has been proved that these unusual acts are directly dependent upon the presence in the blood of special internal secretions produced by the ovaries. This is one of the clearest examples of the determining influence that the chemical composition of the blood may have on mental processes. We spoke of the mind as the government of the body, but it should be remembered that the govern-