Studies with thyroxin carried on in recent years have rounded out the whole concept of the business of the thyroid in the body economy. One may sum it up by saying that the thyroid secretion is the _great controller of the speed of living_. The more thyroid one has, the faster one lives; the less one has, the more slowly one lives. That is not to imply any direct proportion between the amount of thyroid secretion in an individual, and the length of life to which he is destined. The speed of living, in the chemical sense (which is the fundamental sense), and the rate at which the chemical reactions go on that constitute the process of life, are dependent upon the thyroid. When the reactions go faster, more oxygen and food material are burned up or oxidized, more energy is liberated, the metabolic wheel rotates more quickly, the individual senses, feels, thinks and acts more quickly. Likening one energy machine to another, the thyroid may be compared to the accelerator of an automobile. That is a rough and superficial comparison because an accelerator lets in more of the fuel to be burned up, while the thyroid makes the fuel more combustible. It thus resembles more the primer, for a rich mixture of gasoline and air burns at a greater velocity than a poor one. But the action of thyroid could really be simulated only by some substance that could be introduced into the best possible of gasoline mixtures, to increase its combustibility by a hundred per cent or more. For that is what thyroid will do to our food. Nor has it only this destructive or combustion side. Withal there is at the same time a constructive action, for the process frees energy to be used for heat, motion or other need. The thyroid, therefore, in addition to its rôle as an accelerator, acts, too, as the efficient lubricator for energy transformations. So we see it as accelerator, lubricator and transformer of our energies. THE GLAND OF ENERGY PRODUCTION