Dr. G. M. Humphry[144] gives us a memorable study of five hundred old people of over eighty years of age, including an equal number of males and females. He stresses the fact that the descending changes of development are just as orderly as its earlier ascending phases and that civilization enables us to see far more of the natural processes of senescence than was possible when the conditions of life were ruder, for we can now promote the powers of self-maintenance to a degree impossible before. “The chief requisite for longevity must clearly be the inherent or inborn quality of endurance, of steady, persistent, nutritive force, which includes recuperative power and resistance to disturbing agencies and a good degree of balance between the several organs.” That is, each must be sound in itself and have due relation to the strength of other organs. “If the heart and digestive system are disproportionately strong, they will overload and oppress the other organs, one of which will give way.” His findings indicate that both men and women of average size and stature live longer than those much larger or smaller. He thinks, too, that there must be some trait associated with the development of the tubercle bacillus, “which is not only not incompatible with longevity but is not infrequently associated with it”; and this condition he found in eighty-two of the cases he studied. Most of them belonged to long-lived families, had enjoyed good health, appetite, and digestion, had taken little medicine, eaten little meat, been only very slightly addicted to alcohol, had been good sleepers, and rarely suffered from long or exhausting diseases. Most had been much out-of-doors. The average number of teeth in all these subjects was six for men and three for women, and only fifty-seven were entirely without them. The upper or alveolar part of the jaw tended to be absorbed and only the later, firmer growth of the lower part of it to be retained. In primitive man probably loss of teeth would materially shorten life. The skull, which generally becomes lighter, may also sometimes become heavier and increase inwardly as the brain shrinks. The rate of the heart varies very little as age advances. From eighty to ninety years he found it averaged 73–74 beats per minute in men and 78–79 in women. Respiration was 19–20 times per minute and, like the heart, was very slightly accelerated, although the respiratory change might be due to the prevalence of bronchitis in old people.