SCIENCE AND PHILOSOPHY OF RECREATION. 775 



also by cavities at the bottoms of their trunks the heart. The branches 

 of both trees being everywhere hollow, the contained fluid runs up the 

 stem, and through smaller and smaller branches of the arterial tree, 

 into the delicate vessels of the leaves, which may be taken to represent 

 the capillaries. Passing through these into the twigs of the venous 

 tree, the blood returns through larger and larger branches of this tree 

 till it arrives at the trunk, and completes its circuit by again entering 

 the trunk of the arterial tree through the cavities of the heart. Now 

 the blood, in perpetually making this complete circuit of the body, 

 performs three important functions : it serves to carry oxygen from 

 the lungs to all the other parts of the body ; it serves to supply all 

 parts of the body with the nutritive material with which it is charged ; 

 and it serves to drain off from all the tissues of the body the effete 

 products which they excrete, and to present these effete products to 

 the organs whose function it is again to abstract them from the blood 

 and expel them from the body. The two latter functions of the blood 

 those of nourishing and draining I must consider more in detail. 

 They are both performed in the capillaries, so that the object of the 

 arteries and veins may be considered as merely that of conveying the 

 blood to and from the capillaries. Moreover, both functions are per- 

 formed by transfusion through the delicate walls of the capillaries 

 the nutritive material in the blood being thus transfused into the sur- 

 rounding tissues, and the waste product of these tissues being trans- 

 fused into the blood. Thus, in the various vascular tissues there is 

 always a double process going on : first, that of receiving nourishment 

 from the blood, whereby they are being constantly built up into an 

 efficient state for the performance of their various functions ; and, 

 secondly, that of discharging into the blood the effete materials which 

 the performance of these functions entails. Now, when any tissue or 

 organ is in a state of activity in the performance of its function, the 

 activity which it manifests entails a process of disintegration, which is 

 the reverse of the process of nutrition ; that is to say, when a tissue or 

 organ is doing its work, it is expending energy which it has previously 

 derived in virtue of the process of nutrition. Work is therefore, so to 

 speak, the using up of nutrition ; so that if the income of energy due 

 to nutrition is equal to the expenditure of energy due to work, the 

 tissue or organ will remain stationary as regards its capacity for further 

 work, while, if the work done is in excess of the nutrition supplied, the 

 tissue or organ will soon be unable to continue its work ; it will be- 

 come, as we say, exhausted, cease to work, and remain passive until it 

 is again slowly and gradually refreshed or built up by the process of 

 nutrition. Therefoi-e all the tissues and organs of the body require 

 periods of rest to alternate with periods of activity ; and what is true 

 of each part of the body is likewise true of the body as a whole sleep 

 being nothing other than a time of general rest during which the pro- 

 cess of nutrition is allowed to gain upon that of exhaustion. Thus we 



