Page:Popular Science Monthly Volume 15.djvu/794

774 should present what I may call a recreative character, and others not. For it is evident that this character is by no means determined by the relief from labor which these actions or pursuits secure. A week on the moors involves more genuine hard work than does a week in the mines, and a game of chess may require as much effort of thought as a problem in high mathematics. Moreover, the same action or pursuit may vary in its recreative quality with different individuals. Rowing, which is the favorite recreation of the undergraduate, is serious work to the bargeman; and we never find a gardener to resemble his master in showing a partiality to digging for digging's sake. If it is suggested that it is the need of bodily exercise which renders muscular activity beneficial to the one class and not to the other, I answer, no doubt it is so partly, but not wholly; for why is it that a man of science should find recreation in reading history, while an historian finds recreation in the pursuit of science? or why is it that a London tradesman should find a beneficial holiday in the country, while a country tradesman finds a no less beneficial holiday in London? The truth seems to me to be that the only principle which will serve to explain the recreative quality in all cases is what I may call the physiological necessity for frequent change of organic activity, and the consequent physiological value of variety in the kinds and seasons of such activity. In order to render this principle perfectly clear, it will be necessary for me very briefly to explain the physiology of nutrition.

When food is taken into the body it undergoes a variety of processes which are collectively called digestion and assimilation. Into the details of these processes I need not enter, it being enough for my present purpose to say that the total result of these processes is to strain off the nutritious constituents of the food, and pour them into the current of the blood. The blood circulates through nearly all the tissues of the body, being contained in a closed system of tubes. This system of tubes springs from the heart in the form of large, hollow trunks which ramify into smaller and smaller tube-branches. These are all called arteries. The smaller arteries again ramify into a countless meshwork of so-called capillaries. Capillaries are also closed tubes, but differ from arteries in being immensely more numerous, more slender, and more tenuous in their walls. These capillaries pervade the body in such an intimate meshwork that a needle's point can not be run into any part of the body where they occur without destroying the integrity of some of them, and so causing an outflow of blood.

As these capillaries ramify from the arteries, so do they again coalesce into larger tubes, and these into larger, and so on, until all this system of return tubing ends again in the heart in the form of large, hollow trunks. The tubes composing this system of return tubing are called the veins. Thus the whole blood-vascular system may be likened to two trees which are throughout joined together by their leaves, and