The phagocytes may be divided into small active phagocytes, generally known as the microphags, and larger phagocytes called macrophags, which are sometimes active and sometimes still. The former, which are produced in the marrow of the bones, circulate freely in the blood, and occur as some of the white blood corpuscles, or leucocytes. They are distinguishable by their oval shape which facilitates their easy passage through the smaller blood-vessels, and allows of their accumulating in great numbers in the exudations that form around microbes. These exudations may be formed extremely rapidly, and so may arrest infection in the case of many diseases. The absorption of extravasations of blood and the healing of wounds are the work of the macrophags. In a general way, the microphags may be said to rid us of microbes, and the macrophags to heal mechanical injuries, such as hæmorrhages, wounds, and so forth. Macrophags possess a single unlobulated nucleus, and occur as white corpuscles in the blood, lymph, and exudations, or as the fixed cells in connective tissues, the spleen, and the lymphatic glands, &c. [Illustration: FIG. 14. Section of a Renal Tubule, invaded by Macrophags, from the body of an old man of 90 years. _m_ = macrophag. (From a preparation made by Dr. Weinberg.) ] [Illustration: FIG. 15. Cell from the brain of a woman 100 years old being devoured by macrophags. (From a preparation made by Dr. Philippe.) ] The phagocytes are endowed with a sensitiveness of their own, and by means of a sense of smell or taste are able to recognise the nature of their surroundings. According to the impression made upon this sense, they approach the object which arouses it, exhibit indifference to it, or withdraw from its vicinity. When, however, an infectious microbe finds its way into the body, the microphags are attracted by its excretions and swarm into the exudations surrounding it. The macrophags play a very important part in bringing about senile decay. The atrophy of the kidneys in old persons is attributable to their agency (Fig. 14). They accumulate in large quantities in these organs, clustering round about the renal tubes which they ultimately cause to disappear. Having appropriated the place of the renal tubes, the macrophags proceed to form connective tissue, which thus takes the place of the normal renal tissue. A similar process occurs in the other organs that degenerate in old age. In the brains of old persons and animals, for instance, it is known that a number of nervous cells are surrounded and devoured by macrophags (Fig. 15). Judging from the investigations mentioned above, I think I am justified in asserting that senile decay is mainly due to the destruction of the higher elements of the organism by macrophags. This conclusion has been confirmed by means of direct observation, which was the more necessary as it is contrary to the opinions of some biologists.