Page:Popular Science Monthly Volume 39.djvu/791

 somehow more alive than when we see. Apart from sound, the outward world has a dream-like and unreal look—we only half believe in it; we miss at each moment what it contains. It presents, indeed, innumerable pictures of still life; but these refuse to yield up half their secrets."



HE tissues and organs do not all mature at once in man. It results that when we reach mature age our capacity for some exercises has notably diminished, while for others it has preserved its complete integrity. At forty-five years the bones and muscles have lost none of their solidity and vigor. The aptitude for exercises of force and bottom continues. But we can not conclude from this that the man is as apt in all forms of exercise as he was at twenty-five. While the motor apparatus proper is not sensibly modified in the maturity of life, particularly if one has kept it up by regular practice, this is not the case with some other apparatus that begin to decline earlier—notably with that for the circulation of the blood. The heart and the arteries, in spite of the most rational exercises, lose with age a part of their serviceableness, because they lose some of their normal structure.

After thirty-five years of age we recognize, even in conditions of perfect health, a tendency to sclerosis, a defect in nutrition that lessens the suppleness of the vessels and causes them to lose a part of their elastic force. This change, which goes on with increasing age, has received the picturesque designation of the "rust of life." Rust in a machine is the result of a lack of work, while deterioration of the blood-vessels is connected with the working itself of the human machine; it is the result of the wearing out of its most essential wheel-work, and it is to be observed most prominently in men who have carried exercise or work to the point of abuse. All directions for exercise in mature age, all precautions to be taken in its application, are controlled by this great physiological fact of the lessened capacity of the vessels to support violent shocks. This imperfection of the arterial system is the cause of a considerable tendency to shortness of breath; and it is by this shortness of breath that the man's diminished capacity for resistance is shown.

The differences in the structure of the arteries, even though they may not be carried so far as to denote disease, make the man