RELATION OF PHYSIOLOGY TO PHYSICS. 13 



of a constant body-temperature renders physiologically 

 intelligible a large number of phenomena in connection 

 with different bodily activities — for instance, increased 

 metabolism with fall of external temperature, sweating 

 or increased circulation through the skin with muscular 

 work, the relative constancy of metabolism during 

 starvation, and the physiological equivalence of pro- 

 tein, carbohydrate, and fat in proportion to their energy 

 values. These phenomena are intelligible on the assump- 

 tion that warm-blooded animals actively maintain a 

 certain body-temperature, just as they maintain a 

 certain bodily structure and composition. This mode 

 of explanation is not a physico-chemical one, but I 

 venture very confidently to assert that it is a physio- 

 logical one, and in fact the only kind of explanation 

 which really interests and appeals to a true physio- 

 logist. The thread of identity which has been traced 

 through the phenomena just referred to seems to me 

 to have proved a real scientific clue. 



As another example I may perhaps be allowed to 

 refer shortly to the regulation of breathing, as this is a 

 subject on which I have recently been working. Cur- 

 rent accounts of the clock-like action of the respiratory 

 centre during normal breathing, with the expansion and 

 contraction of the lungs acting as a sort of governor 

 through the vagus nerves, always filled me with sus- 

 picion, as it seemed to me that such a regulation was 

 altogether unlike a physiological one. This led me to 

 investigate the matter further, along with Mr Priestley ; 

 and we had the satisfaction of being able to prove that 

 the ventilation of the lungs is actually regulated with 

 exquisite exactness, in such a way as to keep the partial 



