Page:Popular Science Monthly Volume 15.djvu/797

 because these labors require somewhat different faculties of mind for their pursuance.

Before concluding these general remarks on the physiology of recreation, I must say a few words with more special reference to the physiology of exercise. We do not require science to teach us that the most lucrative form of recreation for those whose labor is not of a bodily kind is muscular exercise. Why this should be so is sufficiently obvious. The movement of blood in the veins is due to two causes.

The act of drawing breath into the lungs, by dilating the closed cavity of the chest, serves also to draw venous blood into the heart. This cause of the onward movement of blood in the veins is what is called aspiration, and it occurs also in some of the larger veins of the limbs, which are so situated with reference to their supplying branches that movement of the limbs determines suction of the blood from the supplying branches to the veins. The second great cause of the venous flow is as follows: The larger veins are nearly all provided with valves which open to allow the blood to pass on toward the heart, but close against the blood if it endeavors to return back toward the capillaries. Now, the larger veins are imbedded in muscles, so that the effect of muscular contractions is to compress numberless veins now in one part and now in another part of their length; and, as each vein is thus compressed, its contained fluid is, of course, driven forward from valve to valve. Hence, as all the veins of the body end in the heart, the total effect of general muscular activity is greatly to increase the flow of venous blood into the heart. The heart is thus stimulated to greater activity in order to avoid being gorged with the unusual inflow of blood. So great is the increase of the heart's activity that is required to meet this sudden demand on its powers of propulsion, that every one can feel in his own person how greatly muscular exercise increases the number of the heart's contractions. Now, the result of this increase of the heart's activity is, of course, to pump a correspondingly greater amount of blood into the arteries, and so to quicken the circulation all over the body. This, in turn, gives rise to a greater amount of tissue-change—oxygenation, nutrition, and drainage—which, together with the increased discharge of carbonic acid by the muscles during their time of increased activity, has the effect of unduly charging the blood with carbonic acid and other effete materials. This increased amount of carbonic acid in the blood stimulates the respiratory center in the spinal cord to increase the frequency of the respiratory movements, so that under the influence of violent and sustained exercise we become, as it is expressively said, "out of breath." The distress to which this condition may give rise is, however, chiefly due to the heart being unable to deliver blood into the arteries as quickly as it receives blood from the veins; the result being a more or less undue pressure of venous blood upon a heart already struggling to its utmost to pump on all the blood it can. Training, which is