Now, secretions of the thyroid gland check this aggression of the lower upon the higher cells, as is shown in the studies of cretinism, which is in so many respects nothing but premature old age brought on because the thyroid fluid, the special function of which was to retard this process, was not supplied; and when it fails, old age comes on precipitately and even children often look and act much like prematurely old men and women. On the other hand, the Metchnikoff ferments of the large intestine weaken the higher cells and leave them with less power of resistance, so that they become more easily the prey of the lower cells of the connective tissue. The enemy, however, for the endocrinologist is not primarily a microbe entering from without but a more formidable and subtle foe that springs up within. The difficulty in meeting the situation is immensely enhanced by the fact that the cells of the conjunctive tissue are not only useful but indispensable for the work and the development of every organ at first and continue to be so as long as they do not transcend this their original function and trespass outside it. The white corpuscles, although the source of the connective tissue cells, are themselves our chief defenders. It is they who attack and devour invading microbes but they consume not only these but also higher cells that have, by the action of microbes or otherwise, become debilitated. They are, however, on the whole, so serviceable that we cannot intervene against them but only against our more dangerous and insidious enemy, the conjunctive tissue cells. Not only the thyroid but yet more the tiny parathyroid glands secrete a fluid, the absence of which brings convulsions and death. A knowledge of the function of these glands, no larger than a pinhead, as well as that of the adrenals or of the far more complex pituitary body (hypophysis), each lobe of which plays its own particular rôle, has been nothing less than revolutionary. The effects due either to excess or deficit of the secretion of these glands, which have been studied experimentally in animals and observed in man, show that they have great power even to arrest or accelerate growth itself. It is they that do much to keep us young or make us old. In a sense they furnish the power that makes about all the organs do their work efficiently, as an electric current from a battery may start, or its absence stop, the most diverse kinds of electric machinery. Thus glands have come to play a great rôle in physiology, medicine, and even psychology, and their activities have come to be recognized in very many phenomena both of health and disease, which till recent years no one had suspected. Some of these glands contain a relatively small number of cells but do a vast amount of work and manufacture fluids that no chemist can duplicate and that seem almost magic in their effects. We owe to them growth, health, and vitality.