SCIENCE AND PHILOSOPHY OF RECREATION. 777 



because these labors require somewhat different faculties of mind for 

 their pursuance. 



Before concluding these general remarks on the physiology of recre- 

 ation, I must say a few words with more special reference to the physi- 

 ology 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 capil- 

 laries. 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 propul- 

 sion, 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 drain- 

 age 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 mate- 

 rials. 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, hoAvever, 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 



