Although the hairs of old people grow, they become white, which is a phenomenon of senile degeneration. Although they increase in length, the colouring matter in them becomes reduced and finally disappears. In the “Nature of Man” I described the process by which this blanching takes place, and which may now be regarded as definitely proved. It is useful as a means of interpreting the real nature of the process of senescence. In several published works, I have explained my belief that just as the pigment of the hair is destroyed by phagocytes, so also the atrophy of other organs of the body, in old age, is very frequently due to the action of devouring cells which I have called macrophags. These are the phagocytes that destroy the higher elements of the body, such as the nervous and muscular cells, and the cells of the liver and kidneys. This part of my theory has encountered very strong criticism, especially with regard to the part played by the macrophags in the senescence of nervous tissue. Neurologists in particular, have criticised my interpretation. For several years M. Marinesco[8] has attacked my theory of the atrophy of the nerve-cells in old age. In the first place, he has stated that in old people, and even if these are very old, it is rare to find phagocytes surrounding and devouring the cells of the brain. In support of this contention, he has been good enough to send me two preparations made from the brains of two very old persons. After careful examination I was convinced that my opponent had been inexact. In the brain of the two centenarians (one of whom died at the age of 117 years) there were very many nerve-cells surrounded by phagocytes and in process of being destroyed by them. It happened, however, that as the sections were very weakly stained, it was more difficult to observe the facts than in the preparations upon which I had made my own observations. I have already recorded this fact in the second and third French editions of the “Nature of Man.” Without taking notice of my reply, M. Marinesco has published another criticism of my theory in an article[9] entitled “Histological Investigations into the Mechanism of Senility.” In that work, although he himself had invented the designation “neuronophag” for a phagocyte that devours nerve-cells, he denies the existence of such a power. He thinks that nerve-cells atrophy independently of the cells that surround them. The latter, the so-called neuronophags, only contribute to the atrophy inasmuch as they press against the nerve-cells and deprive them of nutrition. He is confident that the constituent parts of nerve-cells are never found in the neuronophags. There is no question of phagocytosis, of the existence of cells that devour their neighbours.