The retrogressive changes of advancing years can not be rightly termed disease, yet it becomes necessary to regard them as such, for the senile changes, as we shall see, may be but the advanced stages of true arteriosclerosis. Much depends on the nature of the arterial tissue and much on the factors at work tending to injure the tissue. A man of forty years may therefore have the calcified, pipe stem arteries of a man of eighty. Our parents determine, to great extent, the kind of tissue with which we start life. The arteries are elastic tubes capable of much stretching and abuse. In the aorta and large branches there is much elastic tissue and relatively little muscle. When the vessels have reached the organs, they are found to be structurally changed in that there is in them a relatively small amount of elastic tissue but a great deal of smooth muscle. This is a provision of nature to increase or decrease the supply of blood at any point or points. The aorta and the large branches are distributing tubes only. It is after all in the arterioles and smaller arteries that the lesions of arteriosclerosis do the most damage. A point to be emphasized is that the whole arterial system is rarely, if ever, attacked uniformly. That is, there may be a marked degree of sclerosis in the aorta and coronary arteries with very little, if any, change in the radials. On the contrary, a few peripheral arteries only may be the seat of disease. A case in point was seen at autopsy in which the aorta in its entirety and all the large peripheral branches were absolutely smooth. In the brain, however, the arteries were tortuous, hard, and were studded with miliary aneurysms. It is not possible to judge accurately the state of the whole arterial system by the stage of the lesion in any one artery; but on the whole one may say that an undue thickening of the radial artery indicates analogous changes in the mesenteric arteries and in the aorta.