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The desire to live long and to live healthfully is perhaps universal, and leads naturally to an inquiry as to the probable duration of life. In estimating this we must be guided, 1, by our knowledge of the structure and functions of man; and 2, by the history of the lives of those who have lived longest and most free from disease. It is true that very many of such instances may be regarded as exceptional, and that perhaps the ages recorded are in many cases greatly exaggerated; but, making due allowance for all this, there remains a sufficient number of well authenticated instances of great longevity to encourage the belief that the usual term of human life is far below that which men are

capable of attaining. It may perhaps be true that not more than one person in 10,000 attains the age of 100; but it must not be forgotten that at least half of the number probably die in infancy, and that of the remainder, perhaps, not more than two-thirds attain maturity. Then, let us enquire how many of those who attain a healthful maturity, die, in the course of nature, of old age. Setting aside the large number who fall victims to accident, violence, epidemic or contagious diseases, circumstances which must be regarded as occasionally inevitable, the greater number of mankind die from disease, that is, become the victims of their own faults or follies; or sink into early graves in consequence of natural feebleness of constitution, or of some hereditary tendency to disease, that is, in consequence of the vices or follies of their ancestors. They who do not inherit or acquire disease, and who therefore can attain the natural limit of their existence, are a comparatively small number; yet it is this small number only that we have to consider when we ask ourselves— What is the extent of longevity which a healthy individual may hope to attain ? And how may he be best enabled to enjoy life, even until its close ?

Longevity simply implies length of life, the greatest extent of time to which an individual should live, if no accident supervene to cut short his career, or the

occurrence of any disease induce premature decay and death. This must necessarily be a variable period; races, nations, families, individuals, differ as much from each other in longevity as in feature or character. Yet for each the same question will arise—How long may a healthy individual hope to live? Does the extent of the time depend in any degree upon himself, or the circumstances in which he is placed ?—I believe that the answer should be in the affirmative, and will endeavour to show this; but there are others, whose opinions demand great respect, who take a widely different view of the subject; who believe "that a certain stock of vital power is imparted to the embryo at its first formation, as a provision for carrying it through its destined course of existence. In every act of the system a portion of this power is expended, and the greater the expenditure the less must there be remaining, till at length, the whole being consumed, all movements cease, like those of a watch which has run down, and of which the main spring has ceased to act." {Cyclopaedia of Medicine, art. "Age.")

It seems to me impossible to entertain so mechanical an idea of the vital power. If it were true, the duration of each life would depend mainly on the greater or less degree of energy and activity in the individual, the slothful being who merely lived to eat and drink,

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sleep and digest, who avoided all active exertion of body or mind, would surely wear out but slowly the stock of vital power, and attain to an advanced age ; whilst he who for his own good and pleasure, and the advantage and happiness of those around him, availed himself continuously of his great capabilities, actively employing the powers of body and mind, and yet duly recruiting them by food and rest, would much more quickly exhaust the quantum of vital power originally given, and sink into an early grave; the watch having run down sooner, because it was going quicker. But is this the case? Do slothfulness of body and inactivity of mind conduce to longevity? or is not the active employment of all the powers with which humanity is endowed, needful to the preservation of health ?1 If the infant were indeed endowed with a definite quantum of vital power, which it required a certain number of years to wear out, such vital power would be greatest when most accumulated; consequently greatest in the state of infancy, and the vital power of the infant, that is to say, the power of resisting hurtful or noxious agencies, as cold, miasmata, poisonous substances, or impure atmospheres, would be greater in the child than in the man. But is this so? It is somewhat difficult to understand how the body could advance

Dr. Rush observes that he never knew an idler long-lived.

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from infancy, increasing daily in development, size, and strength, although losing daily a portion of vital energy. In order to carry out the hypothesis, we must imagine either that, as with each day of life a portion of vital power is used up, the remaining stock acts with less force; and if so, man is strongest the first day of his existence, and becomes gradually weaker and weaker until its close; or we must suppose this vital power to be enabled, by some unknown arrangement of the machinery, to act, as in the watch, with equal force at all times; and, if so, the vital power would be equal at all periods of life, from infancy to age. But is this so? But the vital power, or at least its activity (whatever it may be), depends as much on the processes of the animal economy, as they on it. It would become feeble without activity; it would sink without nutrition, and its intensity depends then nearly as much on the conduct of the individual, as the due performances of the processes of the economy depend on it. Let us dismiss then the idea of a certain endowment of vitality, and rather believe that the duration of human life is altogether unfixed and uncertain, but dependent on the integrity of the organs of the body and the due and regular performance of their functions.

But what is the natural period of the duration of human life, that period to which a healthy individual

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