The activity acquired by macrophags during old age is closely connected with the phenomena that are characteristic of certain chronic complaints. Sclerosis in old persons belongs to the same category as organic sclerosis, which may be set up by various morbid influences. The analogy between senile decay of the kidneys and chronic nephritis, commonly called interstitial nephritis, is incontestable. The destruction of nervous cells through the agency of macrophags, which we have already mentioned as occurring in old age, is equally a symptom of several diseases of the nervous centres, such as general paralysis of the insane. Arterial sclerosis in old persons is actually an inflammatory disease, similar to the inflammation of the arteries set up by other maladies. The similarity between senility and disease has long been recognised, and partly accounts for the repugnance we all experience at the approach of old age. In childhood and early youth people regard themselves as older than they really are, and long to be “grown-up,” but having once arrived at man’s estate, they do not wish to grow old. An instinctive feeling tells us that there is something abnormal in old age. It cannot be regarded as a part of healthy physiological function. No doubt, because old age is the inevitable lot of mankind, it may be termed normal, in the same fashion as we call the pains of childbirth normal, since few women escape them. In both cases, however, we have to deal with pathological rather than physiological conditions. Just as every effort is made to relieve the sufferings of a woman in labour, so it is natural to try to suppress the evils accompanying old age, but whereas in childbirth an anæsthetic affords relief, old age is a chronic malady, a remedy for which is much harder to find. We have seen that in old age a struggle takes place between the higher elements and the phagocytes, the end being usually a weakening in vitality of the former, while the activity of the latter is enormously increased. It would appear, arguing from this, that one means of fighting against old age, pathologically speaking, would be to strengthen the higher elements of the organism, and to weaken the aggressive capacities of the phagocytes. Let me at once warn the reader that this is not presented as a definite, but as a possible solution of the problem, and is offered for consideration like many other hypotheses on scientific questions. The properties of cellular elements are easily changed when subjected to various influences, and it is therefore not irrational to seek some means of strengthening the blood corpuscles, nerve cells, liver cells, muscular fibres of the heart, and so forth. The task has become easier since the discovery of serums that have specific actions on the tissues.