774 THE POPULAR SCIENCE MONTHLY. 



suits 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 re- 

 semble 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 his- 

 torian 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 Lon- 

 don ? 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 pro- 

 cesses which are collectivelv 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 count- 

 less 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 per- 

 vade 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 destroy- 

 ing the integrity of some of them, and so causing an outflow of blood. 



As these capillaries ramify from the arteries, so do they again co- 

 alesce 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 



