From this point of view it becomes permissible to speak of the five Endocrine Epochs. Similarities and resemblances of mind and body between people at a given period of life, childhood, youth, maturity must be put down to their common government by the salient endocrine of the epoch. So one may list:
Infancy as the epoch of the thymus
Childhood as the epoch of the pineal
Adolescence as the epoch of the gonads
Maturity as the epoch of whatever gland is left in control as the
result of the life struggle.
Senility as the epoch of general endocrine deficiency.
Infancy as the epoch of the thymus explains why, in any given geographic locality, the babies look alike and act alike. Specialists in the observation and treatment of infants have noted that not until after the second year is any tendency to differentiation discernible to any extent among them. It is only after the second year, or somewhere around that time, that the child begins to individuate, and distinct individual traits and a personality manifest their outlines. The thymus is the great inhibitor of all the glands of internal secretion. By its checking activity upon the other members of the endocrine system, the thyroid and pituitary in particular, it gives the baby time to grow in bulk, which is its chief business during the first two years of its existence. It quadruples its birth weight. The brain and nervous system complete their growth in mass by the end of the fourth year. Recall the experiments of Gudernatsch working with tadpoles, who showed that feeding with thymus produced giant tadpoles whose metamorphosis into frogs was inhibited, while feeding thyroid produced frogs the size of flies. Differentiation occurred without the preliminary increase in mass usual. As differentiation and bulk thus appear antagonistic, at least at the beginning of growth, the function of the thymus, at a maximum during infancy, seems then to be to restrain the differentiating endocrines, until sufficient material has been accumulated by the organism upon which the differentiating process may work.
After the second year, the thymus begins to shrink. That is to say, officially its involution begins. Careful dissection will demonstrate some thymus tissue even in a normal subject up to the fourteenth year. This refers to the average normal, for the large thymus may continue large and grow larger after the second year in the type of individual designated in a preceding chapter as the thymocentric.
If the thymus retrogresses after the second year, what takes its place as a brake upon the forward driving impulses of the other endocrines? We have every reason for assigning that rôle to the pineal. It performs its service mainly, in all probability, by inhibiting the sex stimulating effect of light playing upon the skin. Since it is especially a sex gland inhibitor, the thyroid and pituitary become freer to exert their influences than under the thymus régime. And so we find that it is after the second year that thyroid and pituitary tendencies manifest their effects. The Pineal Era, from the second to the tenth to fourteenth years, remains to be investigated from a number of viewpoints interesting to the parent, the educator, and the student of puericulture. Precocity is directly related to early involution of the pineal. For just as the thymus involutes at the second year, the pineal atrophies before the onset of adolescence.
Adolescence is the period of stress and strain throughout the somatic and psychic organism because of the volcanic upheavals in the sex glands. The history of the individual is dominated by them up to twenty-five or so, when maturity commences in the sense of a relative sex stability. They continue to exert a powerful pressure throughout maturity. But life episodes and crises, diseases, accidents, and struggles, experiences of pleasure and pain, as well as climatic factors, settle finally which endocrine or endocrines are left in control as a consequence of the series of reactions the period of maturity may be analyzed into.
THE INTERPRETATION OF SENILITY
Senility inevitably follows maturity, not as night follows day by a mathematical necessity, but because of the process of degeneration which ultimately overtakes all the glands of internal secretion, dominant as well as subordinate. Just why the degeneration must occur no one can say. Injury to the endocrine organs of one sort or another, ranging all the way from emotional exhaustion to bacterial infection, is the reason usually considered sufficient. Just why recuperation and regeneration do not preserve them in the elderly as they do in youth is a problem to be solved when we understand the laws of regeneration, at present almost totally beyond our control. Some say that it is a matter of the wear and tear of our blood vessels, those rubber-like tubes which transport food and drainage with nonchalant equanimity to all cells as long as they last. In the classic phrase: a man is as old as his arteries, ergo his ductless glands will be as old as their arteries. And the age of arteries is simply a matter of wear and tear, the resultant of the function which is universal among molecules. Arteriosclerosis, the hardening of arteries, might be the whole story.
But there are certain experiments and considerations which rather confute that easy explanation, or at least make clear that the mystery is not so simple. The work of Steinach, a Viennese investigator, has contributed most to the elucidation of the nonarterial factor in senility. No one has asserted more loudly the importance of the interstitial cells that fill in the spaces between the tubules of the testes in the male, and the follicles of the ovary in females. Rats have been his medium of study, for they are most easily procurable, live fastest, breed, and withstand experimental and operative procedures better than any other animal.
An old rat is like an old man in his dotage. His bald, shrivelled skin covers an emaciated body. His eyes are dimmed by cataracts and his breathing is labored and difficult because his heart muscle has lost its tone. Huddled in a corner, life to him has become concentrated into the desire for a little food, and immobility. If now, something is done to his sex apparatus, a marvelous transformation may be effected. That something no one could predict. It consists in slitting the genital duct, which leads from the germinal cells to the exterior. After the operation, the germinal cells, which grow into the spermatozoa, atrophy and disappear, since they can no longer function. As if released from some restraint, the interstitial cells, however, multiply enormously. With their multiplication, the miracle of rejuvenation is performed.