Since we have spoken of chemical control it will be appropriate to close this chapter with just a further word about it. In addition to the regulation of smooth muscle and gland activity there are a number of other bodily processes which are affected more or less through chemical agents brought to them by the blood. Many of these processes are of a kind that cannot well be controlled through the nervous system. For example, growth is something which goes on very slowly and yet which distinctly requires some kind of control. It has not perhaps occurred to us that it is more than chance that our ears are substantially the same size, our arms about the same length, our legs the same length, our feet about the same size, and so on. Yet if we pause a moment we realize that as the ears grow there would seem to be no particular reason why they should both stop when they have reached the same size, unless there is some definite regulation. This regulation is present in the form of chemical substances. Of late years a very large amount of study has been given to the materials which have the power of acting as chemical regulators, and a good deal has been learned about them, although there is every reason to believe that a great deal more remains to be found out. It has been necessary to invent a name for substances which act as chemical regulators, because, without such a name, talking about them becomes too cumbersome. The name that has been adopted is _hormone_, from a Greek word meaning to arouse or to stimulate, referring to the ability these substances have of acting upon living cells. Thus far we have spoken of hormones only in connection with their ability to excite those nerve cells by which smooth muscles and glands are controlled. As a matter of fact, their action is much wider than that. They have the power of affecting the metabolism of very many body cells. Some of them appear to have the special function of regulating the rate of metabolism. These we shall talk about in detail, when we return to the study of metabolism in a later chapter. One of the hormones is interesting in connection with the relation of emotions to smooth muscle and gland activity. This is a secretion manufactured by a pair of small glands located in the abdomen near the kidneys. They are known as the _adrenal bodies_. These glands secrete a substance to which has been given the name of adrenalin, which has been shown to be a very efficient chemical excitant to a number of bodily processes. We are particularly interested here in a property it has of arousing smooth muscles and glands in precisely the same way as they are aroused in connection with the disagreeable emotions, such as fright or anger. For example the injection of a little of this adrenalin into the veins of a person or animal will cause his heart beat to be quickened, the blood flow into the brain and the muscles to be increased at the expense of the flow through the skin and abdominal organs, and the other effects to occur that were described above as accompanying these emotions. These facts about the effects of adrenalin had been known for some time before it was realized that they fit in with the reaction by which the body prepares itself to operate efficiently in time of stress. Now we have learned to look upon adrenalin as the “emergency hormone,” meaning that it is a chemical substance which does the same things to the body that are done through the emotions in time of emergency. We look upon this as a reenforcement of the nervous action, indicating the efficiency with which the fitting of the body for time of stress works out. The adrenal bodies are themselves glands and are acted upon like other glands through the part of the nervous system that we have just been describing. In time of emergency a stream of nervous discharges pours into them; they secrete an abundant supply of adrenalin which passes out into the blood and is carried by the blood to all parts of the body, exerting its special functions wherever it comes. There is no doubt that the ability of the body to care for itself in time of need is helped by the outpouring of adrenalin. In one respect, however, this arrangement does not work out perfectly. This is because chemical substances that are poured out into the blood cannot be gotten rid of instantly, and are likely to linger for a shorter or longer time after the actual emergency subsides, so the bodily effects persist for a time after the occasion for them has disappeared. We are all familiar with the fact, for example, that the heart goes on thumping for a long time after a sudden shock.