It does appear, however, that self-indulgence of various kinds is apt to lead to loss of elasticity. For example, even the moderate use of alcohol is now generally recognized by the medical profession as a cause of impairment of elasticity in the arteries. It is probable that intemperance in the use of various foods and drugs leads also to this same condition. We have just seen that the heart is obliged to maintain high blood pressure in order to force the blood through the tiny capillaries. It will be clear that the actual amount of pressure will depend in part upon how much blood is forced through in a minute and in part upon the extent to which the capillaries offer resistance. It is a familiar law of friction that the smaller the tube the greater will be the resistance it will offer to the passage of liquid through it, so that if the capillaries change in size their resistance to the flow of blood through them is bound to vary. The walls of the capillaries are very sparsely provided with muscle fibers, but the very finest subdivisions of the arteries, which are really no larger in diameter than the capillaries, have much more smooth muscle in their walls. These muscles, as we have already seen, can by their contraction or relaxation make the tiny vessels smaller or larger. We have examples of this in the flushing and pallor of the skin. What we wish to do now is to show how the flow of blood through different parts of the body is controlled by changes in the caliber of these tiny tubes. The muscles in the vessel walls have the double nervous control commonly found in smooth muscles, and both sets of nerves trace back to centers in the brain stem. One of these centers causes the muscles to contract and the vessels to become smaller; the effect of this is, of course, to increase the resistance to the passage of blood through them. The nervous center which brings this about is called _vasomotor_, or, more properly, the _vasoconstrictor_ center. Vasoconstriction means literally causing contraction of the vessels, which is exactly what this center does. Not all the blood vessels in the body are acted upon through the vasoconstrictor center. Those of the skin and of the abdominal organs are under its control, but those of the skeletal muscles are not. The result of activity of this center, then, is to make it more difficult for blood to flow through the skin and through the abdominal organs, but the ease of flow of blood through the muscles is not affected. Of course, it will follow automatically that the blood stream will be diverted in a large part from the former regions into the latter. The skin and abdomen together make up so large a part of the whole body that marked constriction of the blood vessels in these two regions is bound to cause a considerable increase in blood pressure.