58 ANIMAL BIOLOGY. [Part I. 



assuming that, either as the result of evolution, or in some other 

 manner at present unknown, pleasure is associated with such 

 activities as are for the good of the organism and of the race to 

 which it belongs, and that pain is associated with such activities 

 as are harmful to the organism or the race. 



Leaving this aspect of general physiology, let us now note 

 that to enable sense organ, nervous arc, and muscle (which, to 

 avoid repetition, we may call the senso-motor arc) to fulfil their 

 physiological function many other processes must go on within 

 the body. For they are living structures, and their life must 

 be maintained. The body is so often likened to a cunningly- 

 wrought piece of mechanism that there is some danger of losing 

 sight of this fundamental distinction between organism and 

 mechanism. In a machine, such as the steam-engine, fuel has 

 constantly to be supplied, and it is by the combustion of the 

 material thus introduced that work can be done by the 

 mechanism. So too in an organism food has constantly to be 

 taken into the body, and it is by the slow combustion of the 

 material thus introduced that work can be done by the animal. 

 So far there is analogy. But in the case of the engine, the com- 

 bustion of the fuel is restricted to a certain part of the machine, 

 and the other parts undergo no more waste of their substance 

 than is caused by friction. In the case of the organism, on the 

 other hand, combustion is not restricted to any spot, but goes on 

 within the working parts, which parts, in and through their 

 very activity, are constantly wasting away, and therefore need 

 constant repair. So that if we liken a nerve to such a mechanical 

 contrivance as a telegraph wire, we must not forget that in 

 transmitting a wave, the nerve is partially destroyed, and there- 

 fore needs constant repair. In this it differs entirely from the 

 telegraph wire ; and in this difference we have a fundamental 

 distinction between organism and mechanism. 



The parts of the senso-motor arc are, therefore, constantly 

 undergoing waste due to oxidation. Hence the necessity of a 

 blood circulation (1) to renew the wasted substance, (2) to carry 

 off the waste products, and (3) to supply the oxygen required 

 for further activity through oxidation. 



