The importance of a healthy life with plenty of fresh air, sunshine, exercise, proper diet, and absence of worry hardly needs insistence. Leonard Williams[79] has epigrammatically summed up the hygiene of old age as “Fresh air, meagre fare, freedom from care.” V THE CAUSES OF SENESCENCE AND GENERAL ATROPHY: LIMITED VITALITY OF CELLS--INFLUENCE OF THE ENDOCRINE GLANDS--METCHNIKOFF’S PATHOLOGICAL EXPLANATION SENESCENCE AND CARCINOMA The causation of old age is so complex that it is not possible to argue with any conviction that any one of the following explanations exclusively solves the problem satisfactorily. LIMITED VITALITY OF THE CELLS To explain the atrophic processes normal to old age it has naturally been assumed that the constituent cells are endowed with a certain store of vitality for themselves and their descendants, and that as this becomes exhausted the process of involution begins. This conception of an inherent limit of life is probable from the analogy of the existing limit of height and size, though in certain plants and trees growth in size and length of days do not show a natural limitation. The process of ageing of the cells is thus regarded as just as much part of their development or life cycle as are their earlier and progressive stages. The exception is that the reproductive cell when it meets with its complement--the ovum with the spermatozoon--and as it were with new blood, starts a fresh lease of life, and so in Sir Edward Sharpey-Schafer’s[80] words “we can only be immortal through our descendants.” That there is a more or less definite cycle during which the cells multiply and after which they cease to do so receives support from the anatomical and physiological changes in old age; thus the ovary undergoes fibrotic atrophy and ceases to be actively functional after the menopause. The atrophy or hypoplasia of the organs is due to diminution of the proliferation of the constituent cells. This is well shown in the lymphatic glands and spleen which are so active in youth and atrophied in advanced life. Salimbeni and Gery[81] point out that the hypoplasia in the woman aged 93 whom they minutely examined was most prominent in the lymphatic tissues and the bone marrow which have to supply the cells most constantly needed. Even in the lowest forms of life senescence and death would occur in the absence of the rejuvenescence associated with reproduction. In the complex congeries of cells making up the higher animals and man this rejuvenescence does not accompany reproduction, and the vitality of the constituent cells diminishes until eventually atrophy and death supervene.