83 ORGANISM AND ENVIRONMENT 



and controlling these activities are the instinctive ex- 

 citatory or inhibitory impulses which we know as 

 hunger, thirst, satiety, discomfort and comfort. These 

 impulses may be regarded as expressions of the many- 

 sided activities which are all directed towards keeping 

 the internal and external environment constant. 



On examining the forms which vitalism has taken 

 we find that the vital principle has been commonly 

 regarded as an influence which resists the tendencies 

 of physical and chemical influences to produce disin- 

 tegration of the body structure. The great chemist 

 Liebig, for example, looked at the oxidation processes 

 in the body from this point of view, and regarded the 

 vital force as something protecting the structure of the 

 body from becoming the prey of oxidation. 



But let us examine the whole matter more closely. 

 It is quite evident that the activities of the various 

 parts of the body are not merely in the direction of 

 maintaining the internal environment constant, but 

 also in the direction of disturbing it. The muscles by 

 their activity may be engaged in obtaining nutriment 

 for the body, but they are also consuming this nutri- 

 ment wholesale. The kidneys are not merely remov- 

 ing superfluous or harmful material from the blood, 

 but they, too, are consuming oxygen and other sub- 

 stances, and producing CO2 and other metabolic 

 products. This is also true even of the lungs and the 

 respiratory centre, for the respiratory centre is vio- 

 lently excited by the products of its own oxidation 

 if its blood supply is checked. Now when we examine 

 those activities which tend to disturb the internal 



