Prolongation of life towards its limits may be healthy or at tended by morbid manifestations. Healthy or normal old age is attended by diminution in functional activity which corresponds with the characteristic changes of atrophy and involution in the structure of organs and tissues and of a diminished area of the capillary blood vessels. With the waning of sexual power the mind becomes calmer and more philosophic, so that the limita tions of activity are more cheerfully accepted; but fatigue is more readily produced, memory, especially for names, becomes less agile, and will power, like the gait, becomes less certain. The mental outlook is largely determined by that of the individual in the past, happy and kindly, or pessimistic and uncharitable. Often there is a tendency to ego-centricity and to nervous appre hension, as the preacher (Koheleth) in Ecclesiastes said "Fears shall be in the way." It may be pointed out that the glands of internal secretion become old with the rest of the body, not that their atrophy causes old age. For it has been thought that senescence results from failure of the thyroid, and especially of the interstitial cells of the testes, and experiments and operations by Steinach and Voronoff to supplement this want have been fol lowed by dramatic, if temporary, rejuvenation, which in animals at least cannot be explained by suggestion ; but that life is thus prolonged as well as being made to burn more brightly remains to be proved in the case of man.
It is hard to draw a sharp line of demarcation between normal old age (senescence), with its limitations due to atrophic changes in the body, on the one hand, and on the other hand the mani festations in the elderly resulting from past infections and diseases (senility). Physiological old age, that is freedom from any pathological change, is agreed to be rare, and it has been said by Terence, Cicero, Sanctorius, and often since that old age is itself a disease. Eli Metchnikoff attributed the senile accom
paniments of advanced years to pathological and preventable causes, especially poisons produced by bacteria in the large in testine, these toxins causing degeneration of the bodily cells which are eaten up by more resistant cells called macrophages; this hypothesis has been much discussed and is still (1928) being investigated. That disease may produce the picture of morbid old age prematurely is well established, and in the remarkable but rare condition of progeria this occurs in childhood. The various parts of the body do not all grow old at the same time, but when these changes are much more advanced in a vital organ, especially the brain, than elsewhere, the proper harmony of the system is so dis turbed that the condition becomes pathological, and then instead of a happy, healthy and pleasant old age there is the picture of incapacity, pain and misery so that existence becomes a burden to the individual and to his family.
Some diseases are specially prone to attack the aged, though very few are entirely confined to the evening of life. These are mainly of a chronic character due to degeneration, especially changes in the arteries of the brain and spinal cord producing mental disturbance, tremors, and paralysis of various kinds. The acute fevers, such as measles, typhoid, scarlet fever and diph theria, rarely attack the old, probably because immunity has gradually been acquired, but pneumonia and erysipelas are ex ceptions to this rule, and pneumonia, with but few symptoms, often proves to be the last straw in breaking the feeble thread of existence.