In any individual instance the exact line which separates healthy old age (senescence) from old age complicated by a morbid process, _i.e._ by some factor other than the gradual atrophy and restriction of functional activity, or senility, may be difficult or impossible to draw. The dictum of Terence, Cicero, and Sanatorius that old age is a disease probably still finds acceptance with many. It is indeed clear that exposures to infections and poisons would produce changes more easily in cells that are beginning to fail in vitality. Healthy old age should be a normal process of involution with progressive atrophy and loss of vitality, and free from any morbid change due to other factors whether extrinsic, such as infection, or intrinsic and due to abnormal metabolism. As the bodies of the aged usually show a number of changes additional to those of normal involution, some of which, such as arteriosclerosis, are so frequent that they have sometimes been erroneously regarded as part or even the cause of old age, it is essential to recognize and to try to draw a distinction between physiological old age and senility from the effects of disease (_Senium ex morbo_). But about the anatomy and physiology of normal old age much remains to be learnt; more indeed is known about the pathology of the aged, a subject which includes the damage done in the past, perhaps in youth, and morbid processes starting during advanced life. In attempting to decide when old age should be regarded as a disease or merely as a process of involution or retrogression which naturally follows the earlier and progressive stage (youth) of development, it may be well to refer to the meaning of “disease” and “health.” Disease, or want of ease, has been variously defined as evidence of imperfect function, as discord, and as maladjustment between the individual and his environment (Moon[201]), and Health as the indication of perfect functional activity, as harmony between the individual and his environment. In the different stages of life’s cycle there should be a correspondence between the individual’s desires and his powers so that there is harmonious co-ordination; this should hold good in normal old age as it does in youth. The frequent complaints of old people show that there is maladjustment and disease, for if the decline of vitality were uniform throughout the body the equilibrium would, though altered as a whole, still be maintained, and there would no longer be a discordant desire for activity, for which other parts of the body are, from a more advanced state of atrophy or morbid change, unable. Thus it would appear that the conscious disabilities of old age are not the necessary results of a true physiological involution, and that the late Sir Andrew Clark’s definition of Old Age as “the period at which a man ceases to adjust himself to his environment” should be regarded as true of senility or morbid old age but not of senescence or healthy old age.