The eye is an organ that is modified in old age without the action of macrophags. Cataract and the senile arc which appears as a milky ring at the edge of the cornea are frequent in old age. These modifications are due to impregnation of the parts affected by fatty matter which makes them opaque. This deposition of fat[26] has been attributed to defective nutrition. In most organs such fatty degeneration is followed by phagocytosis, but the cornea and the crystalline lens are exempt from this consequence for anatomical reasons. Most organs possess in addition to their higher elements a constant source of macrophags. Such a source of phagocytosis is the neuroglœa in nervous tissues, the sarcoplasm in muscular tissues; the bones contain osteoclasts and the liver and the kidneys are readily invaded by phagocytes from the blood. The lens and the cornea have no cells that are able to become macrophags. [Illustration: FIG. 13.—Bone marrow from the sternum of a man aged eighty-one years. (From a preparation made by Dr. Weinberg.)] Some infectious diseases bring about precocious senility. A syphilitic child is “a miniature old man, with wrinkled face, skin dull and discoloured and flabby and hanging in folds as if it were too large.”[27] In such a case the active agent is the microbe of syphilis which has poisoned the child on the breast of its mother. It is no mere analogy to suppose that human senescence is the result of a slow but chronic poisoning of the organism. Such poisons, if not completely destroyed or eliminated, weaken the tissues, the functions of which become altered or enfeebled, so that, amongst other changes, there is deposition of fatty matter. The phagocytes resist the influence of invading poisons better than any of the other cells of the body and sometimes are stimulated by them. The general result of such conditions is that there comes to be a struggle between the higher cells and the phagocytes in which the latter have the advantage. The answer to the question as to whether our senescence can be ameliorated must be approached from several points of view. This course I shall now follow. PART II LONGEVITY IN THE ANIMAL KINGDOM I THEORIES OF LONGEVITY Relation between longevity and size—Longevity and the period of growth—Longevity and the doubling in weight after birth—Longevity and rate of reproduction—Probable relation between longevity and the nature of the food The duration of the life of animals varies within very wide limits. Some, as for instance, the males of certain wheel animalculæ (Rotifera) complete their cycle of life from birth to death in 50 or 60 hours, whilst others, like some reptiles, live more than 100 years, and quite possibly may live for two or three centuries.