Marinesco,[297] an authority upon everything connected with the nervous system, has disputed my theory, asserting that the destruction of the specific elements in the nervous centres of old persons is not brought about through the agency of macrophags. In support of his theory, M. Marinesco was good enough to send me a series of preparations from the spinal marrow of persons of very advanced years from which evidence of destruction by means of phagocytes or phagocytosis, was completely absent. I freely admit the absence of phagocytosis in M. Marinesco’s preparations, but these were derived from the cells of spinal marrow, which is much less subject to the ravages of senile decay than is the brain. Even in the lower portions of the encephalon, senility and its parallel, phagocytosis, are uncommon, whereas in the brains of old persons, which are more generally affected by senile decay, the higher elements are clearly shown to undergo destruction by macrophags (Fig. 15). The same phenomenon may be observed in the case of parrots and dogs of advanced age, and in other animals. So universal a symptom of old age is the invasion of the tissues by macrophags, that it must be regarded as of immense importance. In order, however, to determine more precisely the nature of the function fulfilled by these phagocytes, it was necessary to select a specially favourable subject of investigation. My choice fell upon an examination into the causes of the hair turning white,[298] that being as a rule the first visible sign of approaching old age. Hair, before it has lost its colour, is full of pigment scattered throughout the two layers of which each hair is composed. At a given moment, the cells of the central cylinder of a hair become active, and proceed to devour all the pigment within their reach. Once they are filled with coloured particles, these cells, which are a variety of macrophag (generally called pigmentophags or more properly speaking chromophags), become migratory, and, quitting the hair, either find their way under the skin or leave the body (Fig. 16). The colouring-matter of the hair is removed in this way by chromophags, leaving the hair colourless. The process by which hair becomes white is of importance, because it shows that the activity of macrophags is a dominant factor in bringing about senile decay. The brittleness of old people’s bones is probably due to a similar cause, _i.e._, to the absorption and destruction of the framework brought about by macrophags invading the layers of bone. There is still much that remains unknown in this subject, which is well worthy of special research. [Illustration: FIG. 16. Hair about to become grey. Chromophags transporting the pigment granules. ]