The World's Greatest Books [Volumes 1-15, 17-20]

I.—Senile Debility

When we study old age in man and the lower animals, we observe certain features common to both. But often among vertebrates there are found animals whose bodies withstand the ravages of time much better than that of man. I think it a fair inference that senility, that precocious senescence which is one of the greatest sorrows of humanity, is not so profoundly seated in the constitution of the higher animals as has generally been supposed. The first facts which we must accept are that human beings who reach extreme old age may preserve their mental qualities, notwithstanding serious physical decay, and that certain of the higher animals can resist the influence of time much longer than is the case with man under present conditions.

Many theories have been advanced regarding the cause of senility. It is certain that many parts of the body continue to thrive and grow even in old age, as, for instance, the nails and hair. But I believe that I have proved that in many parts of the body, especially the higher elements, such as nervous and muscular cells, there is a destruction due to the activity of the white cells of the blood. I have shown also that the blanching of the hair in old age is due to the activity of these white cells, which destroy the hair pigment. Progressive muscular debility is an accompaniment of old age; physical work is seldom given to men over sixty years of age, as it is notorious that they are less capable of it. Their muscular movements are feebler, and soon bring on fatigue; their actions are slow and painful. Even old men whose mental vigour is unimpaired admit their muscular weakness. The physical correlate of this condition is an actual atrophy of the muscles, and has for long been known to observers. I have found that the cause of this atrophy is the consumption of the muscle fibres by what I call phagocytes, or eating cells, a certain kind of white blood cells.

In the case of certain diseases we find symptoms, which look like precocious senility, due to the poison of the disease. 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 got rid of, weaken the tissues, the functions of which become altered or enfeebled in which the latter have the advantage. But we must make further studies before we can answer the question whether our senescence can be ameliorated.

The duration of the life of animals varies within very wide limits. As a general rule, small animals do not live so long as large ones, but there is no absolute relation between size and longevity, since parrots, ravens, and geese live much longer than many mammals, and than some much larger birds. Buffon long ago argued that the total duration of life bore some definite relation to the length of the period of growth, but further inquiry shows that such a relation cannot be established. Nevertheless, there is something intrinsic in each kind of animal which sets a definite limit to the length of years it can attain. The purely physiological conditions which determine this limit leave room for a considerable amount of variation in longevity. Duration of life, therefore, is a character which can be influenced by the environment.

The duration of life in mammals is relatively shorter than in birds, and in the so-called cold-blooded vertebrates. No indication as to the cause of this difference can be found elsewhere than in the organs of digestion. Mammals are the only group of vertebrate animals in which the large intestine is much developed. This part of the alimentary canal is not important, for it fulfils no notable digestive function. On the other hand, it accommodates among the intestinal flora many microbes which damage health by poisoning the body with their products. Among the intestinal flora there are many microbes which are inoffensive, but others are known to have pernicious properties, and auto-intoxication, or self-poisoning, is the cause of the ill-health which may be traced to their activity. It is indubitable that the intestinal microbes or their poisons may reach the system generally, and bring harm to it. I infer from the facts that the more the digestive tract is charged with microbes, the more it is a source of harm capable of shortening life. As the large intestine not only is that part of the digestive tube most richly charged with microbes, but is relatively more capacious in mammals than in any other vertebrates, it is a just inference that the duration of life of mammals has been notably shortened as the result of chronic poisoning from an abundant intestinal flora.

When we come to study the duration of human life, it is impossible to accept the view that the high mortality between the ages of seventy and seventy-five indicates a natural limit to human life. The fact that many men from seventy to seventy-five years old are well preserved, both physically and intellectually, makes it impossible to regard that age as the natural limit of human life. Philosophers such as Plato, poets such as Goethe and Victor Hugo, artists such as Michael Angelo, Titian, and Franz Hals, produced some of their most important works when they had passed what some regard as the limit of life. Moreover, deaths of people at that age are rarely due to senile debility. Centenarians are really not rare. In France, for instance, nearly 150 centenarians die every year, and extreme longevity is not limited to the white races. Women more frequently become centenarians than men—a fact which supports the general proposition that male mortality is always greater than that of the other sex.