of arousing the center whose action speeds up the heart. In at least one case, that of the center controlling the sweat glands, the nerve cells are aroused by an increase in temperature; on a very hot day or when one exercises briskly the temperature of the blood begins to go up; this arouses the center which controls the secretion of sweat; nervous disturbances are poured out to the sweat glands and they become active. The effect of their activity is to remove heat from the body, and so help to lower its temperature. Here we have an example of an automatic regulating device in which nerve cells are controlled through the blood. Except for this one instance of the effect of blood temperature the actions of this class are thought to be wholly by means of chemical substances, and for that reason we speak commonly of the third method of control of the smooth muscles and glands as chemical control. We have then altogether three methods of control; through reflexes, through the emotions, and through chemical substances in the blood. 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