FIGS. 6. & 7.—Two nerve-cells from the cortex of the brain of an old dog aged fifteen years. The neuronophags surrounding the nerve-cells contain numerous granulations. (From preparations made by M. Manouélian.)] The general result of my investigation into the criticisms that have been published on this matter has confirmed me in my belief that neuronophagy plays a most important part in senescence, and recent observations that I have made with M. Weinberg have completely supported this view. The bleaching of hair and the atrophy of the brain in old age thus furnish important arguments against the view that senescence is the result of arrest of the reproductive powers of cells. Hairs grow old and become white without ceasing to grow. The cessation of the power of reproduction cannot be the cause of the senescence of brain-cells, for these cells do not reproduce even in youth. III MECHANISM OF SENILITY Action of the macrophags in destroying the higher cells—Senile degeneration of muscular fibres—Atrophy of the skeleton—Atheroma and arterial sclerosis—Theory that old age is due to alteration in the vascular glands—Organic tissues that resist phagocytosis The instances which I have selected in attempting to describe the mechanism of senescence of the tissues are not the only cases in which the importance of phagocytosis is evident. The blanching of hair is due to the destructive agency of chromophags; in atrophy of the brain neuronophags destroy the higher nerve-cells. In addition to these instances of phagocytosis, in which the active agents belong to the category of macrophags, there are many other devouring cells, adrift in the tissues of the aged, and ready to cause destruction of other cells of the higher type. The phagocytic action is not so manifest as in the case of infectious diseases, partly because it is the method of macrophags to absorb the contents of the higher cells extremely slowly. The mode of action is well seen in the atrophy of an egg-cell (Fig. 8), where the surrounding macrophags gradually seize hold of the granules within it and carry these off. As the process goes on, the ovum becomes reduced to a shapeless mass, and finally leaves only a few fragments, or disappears completely. M. Matchinsky[14] has studied the series of events in my laboratory, and I am myself well assured of the importance of the action of macrophags in the atrophy of the ovary. [Illustration: FIG. 8.—Ovum of a Bitch in process of destruction by Phagocytes, which are full of fatty granules. (After M. Matchinsky.)] The phenomena of atrophy in general and of senile decay afford other cases of tissue destruction in which the phagocytic character of the process is more modified and obscure than in nerve-cells and ova.