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. After some weeks the sluggish currents of being in the rat, which had slowed down as a preliminary to stopping altogether, flow fast and furious. Waves of new chemical substances inundate his cells. And they respond like the fields that border the Nile after the annual flood. All his tissues, skin, muscle, nerve, even bone, are restored. A vitality is created which makes him bound and dart like a youth of his species. In due time, though, senility returns. It is as if a storage battery, recharged, runs down and becomes dead again. Slitting the genital duct of the other testis, causing its interstitial cells to hypertrophy and multiply, repeats the effects of the first experiment. The organism responds again to the new waves of vitality that vibrate through it. That it is recharged is demonstrated again by a revival of sex appetite and sex activity. The female which had become an object of indifference is reinstated as a creature to be sought and pursued. The second period ends in its turn. And now entirely new interstitial glands, in the form of fresh testes removed from a young animal, are transplanted into the body of the old rat. Once more youth returns. But now it burns itself more quickly than even before. An acute exhaustion of the mind appears first. Then all the other phenomena of old age steal back upon the old rat, and senility, firmly established in the saddle, rides him to the end. THE POSSIBILITIES OF REJUVENATION