The impression must not be gained that hypertension in itself always constitutes a disease or even a symptom of disease. Hypertension itself is practically always a compensatory process. That is to say, it is the attempt on the part of the body to equalize the distribution of blood in the body when there is some poison causing constriction of the small arteries. In this sense hypertension is not only essential, but actually life-saving. A heart which is so diseased that it can not respond to the call for increased action by hypertrophy of its fibers, would shortly wear out. The very fact that the heart becomes enlarged and the tension in the arteries becomes high, indicates that in such a heart there was great reserve power. But while hypertension is largely an effort at adjustment among the various parts of the circulation, it nevertheless tends to increase, provided the cause or causes which produced it act continuously. Moreover, as has been said (Chap. II), the arterioles do not respond to increased work on the part of the heart by expanding, but by contracting. A vicious circle is thus maintained which eventually must lead to serious consequences. Hypertension is then, if anything, only a symptom which may or may not demand treatment. That hypertension leads to the production of sclerosis of the arteries has been repeatedly affirmed here. In certain cases it is good and should not be experimented with. In other cases it is bad and some treatment to reduce the tension must be tried. The main point is to regard hypertension as one regards a compensated heart lesion. Prof. T. Clifford Allbutt divides the causes of arteriosclerosis clinically into three classes: (1) The toxic class--the results of poisons of the most part of extrinsic origin, chiefly those of certain infections. In some of these diseases, the blood pressures, as for example, in syphilis, are ordinarily unaffected; in others, as in lead poisoning, they are raised. (2) The class he calls hyperpietic,[15] in which an arteriosclerosis is the consequence of tensile strength, of excessive arterial blood pressure persisting for some years. A considerable example of this class is the arteriosclerosis of granular kidney, but in many cases kidney disease is, clinically speaking, absent. (3) The involutionary class, in which the change depends upon a senile, or quasisenile degradation. This may be no more than wear and tear, a disposition of all or of certain tissues to premature failure--partly atrophic, partly mechanical--under ordinary stresses; or it also may be toxic, a slow poisoning by the "faltering rheums of age." In ordinary cases of this class the blood pressures for the age of the patient are not excessive. Although the toxins of the specific fevers, notably typhoid, as stated above, and influenza, have been shown to produce arteriosclerosis, this, under favorable circumstances he believes tends to disappear. This has been shown by Wiesel.