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

BOOK 1

cold, and drowsy, and finally succumb to sleep. Doubtless other glands are at work, too, but the share of the thyroid is prettily demonstrated by the fact that a single thyroid injection will completely waken a hedgehog from his winter’s sleep.

Half a century ago, nothing was known of the functions of these ductless glands. They were merely puzzles for the anatomist. To-day we know that, including the thyroid and the pituitary body, there are four, perhaps five different glands in our bodies entirely concerned with this business of internal secretion, and two of these five, the adrenal and the pituitary, have a double function and are formed of two distinct parts, each producing one or more quite different hormones. Besides these definite anatomical structures we have named there are several organs which have other functions as well, but which also contain masses of internally secreting cells. Altogether we know now of more than a dozen separate hormones each with its own special task within the body.

Of these hormones, two—secretin and adrenin—co-operate with the nervous system in bringing about rapid variations in the state of our organs in adjustment to the fluctuating conditions to which we are exposed. We have already described the action in Chapter 2 of this Book 1.

Other internal secretions have exclusively chemical functions. The pancreas, for example, has islets of endocrine cells scattered among its digestive tubules, and_ their secretion, zisulin, has a very important function in controlling the handling of sugar by our tissues. The parathyroid glands are small bodies lying at the sides of the thyroid, and until a few decades ago were regarded as mere appendages of that organ. Now we know that they produce a secretion which regulates the amount of lime in the blood. When this is insufficient, defects appear in bones and teeth ; when it is gravely deficient the patient has an attack of muscle-spasms or tetany. In the last few years a way has been found to prepare extracts of the parathyroids, whose injection will relieve the symptoms of this disorder.

The endocrine functions of various other organs of the body have still to be explored. The thymus, for example, is a large, pink gland lying over the heart in very young children but dwindling and disappearing before the age of discretion is reached. Presumably it has an influence on early development.

It should not be thought that the different ductless glands exert their functions in 102

THE SCIENCE OF LIFE

CHAPTER 4

isolation and independently of each other. Often a number of glands affect the same process in various ways—in the case of sugar metabolism, for example, the thyroid stimulates oxidation of sugar, the post-pituitary favours the conversion of sugar to fat, the adrenal medulla brings about liberation of sugar from the liver, and the pancreas

controls the taking-up of sugar by the

tissues from the blood—and thus the various secretions may interfere, antagonizing or reinforcing each other’s activities. Again, the various ductless glands may influence each other directly. There is an intimate interrelation between the adrenals and the thyroid, for feeding dogs on adrenal cortex causes their thyroids to store more thyroxin. Similarly the endocrine parts of the ovary and testis affect other ductless glands, for castration and pregnancy are accompanied by various changes in various glands; and X-raying the testes of some mammals, which causes overgrowth of the internally secreting tissue, produces an apparently degenerative decay in the pituitary. Like other tissues, the ductless glands are sensitive to changes in the composition of the blood, and thus, in a variety of ways, they can interact. Manifestly our present knowledge of internal secretion is still very incomplete, and a good deal of premature speculation—indeed, of nonsense—has been written about it. Nevertheless it is clear that the actions of these glands are of the profoundest physiological importance, and the unravelling and mapping of this tangled skein of chemical actions —Aa process comparable to the mapping of fibres in the nervous system—is one of the most fascinating problems that the physiologist of the next few decades will have to tackle. So we close this succinct review of current knowledge in this field. Beneath the outer appearances of life this interplay of secretions and counter-secretions goes on, like the authors, producers, prompters and so forth of a'dramatic piece. They do not challenge our attention. The play’s the thing. The play is a Cat or a Fish or a Mouse or an Elephant, according to circumstances. The play they put on the stage when the field of action is Homo sapiens has been called by an eminent observer of the species the Seven Ages of Man. Pituitary, pineal gland, thyroid,’ the interstitial cells of testis and ovary, all contribute to the drama, whose first act is ““ the infant mewling and puking in its nurse’s arms” and whose last, the “lean and slipper’d pantaloon,” belatedly in search of Voronoff, Steinach, Norman Haire or whatever other hope may offer.