and, notably, the respiration and the circulation of the blood. But, while work renders these two functions more active, effort, on the contrary, restrains them. By a mechanism we can not study here all intense effort reacts upon the lungs, the heart, and the large blood-vessels. When we try to raise a heavy weight, or to break between our hands a stick that offers a strong resistance, we feel the muscles of the breast and the abdomen hardening and violently compressing the lungs, as well as the heart and the large blood-vessels. Respiration is suspended, the blood flows back toward the veins, and we see them swelling on the neck and forehead. This violent pressure is not always without danger.
We have selected tennis, the most celebrated and the most French of games, as the type of our demonstration. All games in which projectiles are thrown, or the ground is skipped over, are but variants of tennis, and conclusions drawn from it are valid also as to them.
There are numerous other simple and easy games which are none the less hygienic. The most elementary of all, the game of tag, which children improvise as if by instinct—as also do young animals—is not less efficacious than the most elaborate sports to stimulate respiration and the circulation of the blood. It is because these games represent, in the aggregate, much work. At each step in running, the child takes from the ground and lifts to a certain height above it a relatively considerable weight, that of the body. Now, we know that work in mechanics is estimated by multiplying the weight of the mass raised by the height to which it is lifted. Though the body is lifted only a little at each step in running, yet as these steps are renewed as often as four or even six times a second, we see what number of kilogrammetres a game of tag a quarter of an hour in length may represent. This considerable work is accomplished without effort, because the legs, the thighs, and the pelvis, which co-operate in executing it, are re-enforced by the strongest muscular masses of the body. But while the "effort" passes unperceived by the muscles in the running child, the "work" makes its general effects plainly felt in the organism. The least attentive observer has remarked how running accelerates the circulation of the blood, and especially how it stimulates respiration and magnifies the heaving of the ribs, which is the essential cause of the bellows movement which draws the air into the chest. We might say that in the running child the organ that works most is just the one that it is most important to develop, the lung.
It would be superfluous to pursue the analysis further. We have seen that games, although attractive and easy, are not less serious exercises than our methodical analysis, and that they