else arises so far in the future that the immediate bodily changes caused by the worry do no good. Evidently worry is an emotion to be combated. The old adage about not crossing the bridge until it is reached has really a great deal more to do with the preservation of health than most of us realize. We have described the control of smooth muscles and glands through reflexes and in connection with the emotions. We have still to mention briefly a third way in which nervous disturbances leading to these organs may be originated. In the last chapter the brain was described as consisting of three chief parts: the cerebrum, which is the seat of memory and association, and so of all thought processes; the cerebellum, which governs the reflex of locomotion; and the brain stem about which nothing was said except that it is a region through which all nervous disturbances have to pass on their way into or out from the brain. The brain stem has a special function of its own in connection with the control of what are often called the “vital processes.” Since most of these vital processes are concerned with smooth muscle or gland action they are of interest to us here. In the brain stem are located groups of nerve cells to which are given the name of “centers.” From these extend nervous pathways to various bodily structures. For example, there are two centers having to do with the activity of the heart, and both of these have nerve pathways leading to that organ. Over one of them pass the discharges which cause the heart to beat more rapidly, over the other the discharges which slow it down. There are also centers for controlling the caliber of the various small blood vessels and so the amount of blood that flows through the various organs of the body. Still another center has control of the secretion of sweat. These are a few of the centers which are present in the brain stem. It is evident that these centers must be played upon directly from the sense organs in the reflex control of the smooth muscles and glands and also from the cerebrum in their emotional control. The third way in which these centers can be affected is by a direct action of the blood which circulates through them. Of course, every center is made up of nerve cells which are alive and which share with all other living cells the necessity of being sufficiently nourished. This means that there must be small blood vessels here and there among the nerve cells, so that interchanges of material can take place between the blood and the fluid immediately surrounding the cells. Among the materials which may come from the blood are some special chemical substances which have the ability to arouse these cells to activity. For example, the very rapid beating of the heart following vigorous muscular exercise is due, in part at least, to the presence of chemical substances that are poured out into the blood from the laboring muscles and which have the effect