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

BOOK 1

laid down. In the female, the pelvis broadens further, a layer of fat is deposited all over the body that gives it a rounded, graceful contour, and the distinctive feminine figure is accentuated. In the male, there is a growth of hair on the face and the larynx enlarges, causing the “ breaking” and deepening of the voice—processes which are not completed until about the twentyfifth year. In both sexes there are correlated psychical changes: the sexual instincts attain full power and the mind deepens and changes its emotional tone.

Somewhere in the early twenties these adjustments are completed and the body is fully mature. For a number of years reproductive capacity is at a maximum, and then it begins to decline. At about forty-five years, in the female, the monthly crisis ceases suddenly; in the male the ability to produce spermatozoa diminishes slowly. By this time the cycle is completed ; the individual is wearing out, and new bodies have been made to replace it.

The difference between the internal secretions from the testes or ovaries determines the other differences of the sexes. In this respect sexual development is not unique, for the growth and differentiation of the majority of our parts are regulated, in the later stages at least, by internal secretions from such ductless glands as the thyroid and pituitary.

The study of internal secretion is one of the youngest and, at the present time, one of the most fruitful branches of physiological research. We may well close our account of the body by noting some of the most important results of this work, taking first the two glands that play the greatest part in regulating growth.

The thyroid gland lies in the neck. It is a roughly U-shaped body, brownish-red in colour, with two thickened lobes lying one on each side of the larynx and a bridge connecting the lower ends of the two lobes and crossing in front of the windpipe. It contains a number of minute, completely closed vesicles that are full of a yellow fluid. The essential facts about the thyroid have been known for a longer time than about any other ductless gland, owing to the fortunate accident that its secretions pass unchanged through the digestive system into the blood, so that patients can be treated by simply feeding them with dried gland substance. And our knowledge of the thyroid has recently taken an important step forward, because it is one of the two ductless glands whose active principles have been isolated and

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THE SCIENCE OF LIFE

CHAPTER 4

analysed, and even successfully synthesized in the laboratory from simpler materials. The other is the central part of the adrenal gland, of which we spoke in § 2 of Chapter 3. The products in question are called thyroxin and adrenin — respectively. The majority of the internal secretions have not yet been isolated, their presence being inferred from the effects they produce.

The secretions of the thyroid affect profoundly a great number of different physiological processes. One milligram of thyroxin injected into a normal man will cause his chemical activity, as measured by oxygen consumption and carbon dioxide production, to go up by two per cent., although the amount injected is only about one seventymillionth part of the man’s weight. Heavy doses of thyroid speed up the vital engines to such an extent that unless enormous quantities of food are provided there is a breakdown of the substance of the tissues themselves in an effort to get more fuel to burn, and the animal or man wastes away. The substance can also affect temperament ; heavy doses of thyroxin make a perfecily normal man nervous and irritable.

It is clear that a gland like the thyroid may have very marked effects upon all the bodily processes, including growth, if its secretion is for long periods of time a litile above or a little below normal. A disease known as myxoedema is due to deficiency of the secretion in the adult. It is characterized by a sluggishness of the various chemical and physical reactions that go on in the body; the pulse is slowed, the temperature is low, the appetite is blunted, speech becomes a drawl, the mind becomes viscous. At the same time there is a thickening of the connective tissues under the skin, producing a puffiness and yellowness of the face and hands and falling of the hair and a considerable accumulation of fat.

In children with defective thyroids similar phenomena appear, but growth, particularly of the brain and skeleton, also ceases. A child so affected may live for many years, but even when thirty years old it retains a distorted childish appearance, and_ has scarcely the intelligence of a child of four or five. The condition is known as credinism. In most cases cretinism and myxoedema can be cured by administering a regular dose of thyroid extract with the food. It is probable that slight thyroid deficiency is very common, for in many cases administering thyroid to people of middle age has a beneficial effect, reducing corpulence and promoting the growth of hair.