RELATION OF PHYSIOLOGY TO PHYSICS. 3 



can assign to such sources the energy of animal heat, 

 muscular work, glandular, nervous, and other activity : 

 also the carbon dioxide, urea, salts, and many other 

 substances which leave the body or are formed within 

 it. All of this new knowledge may be regarded as 

 progress towards a physico-chemical explanation of life. 

 But there is another aspect to be considered ; for side 

 by side with what I have just referred to there has been 

 a different kind of increase of knowledge with regard to 

 animal metabolism. This growth of knowledge relates 

 to the manner in which the passage of energy and material 

 through the body is regulated in accordance with what 

 is required for the maintenance of the normal structure 

 and activities of the body. In Liebig's time, for in- 

 stance, it was believed that the rate of respiratory ex- 

 change was regulated simply by the supply to the body 

 of oxygen and food material. If one breathed faster, or 

 if the barometric pressure or percentage of oxygen in 

 the air increased, the respiratory exchange was assumed 

 to be also increased, just as ordinary combustion outside 

 the body would be increased by an increased supply of 

 oxygen. If, again, one took in more food, it was supposed 

 that the excess went to increase the rate of combustion 

 in the blood (luxus consumption), just as a fire is increased 

 when more fuel is supplied. We now know that these 

 assumptions were wholly mistaken, and that the re- 

 spiratory movements, respiratory exchange, and cor- 

 responding consumption of food material in the body 

 are regulated with astounding exactitude in accordance 

 with bodily requirements. If, for instance, the body 

 consumes more protein, it economises a quantity of fat 

 or carbohydrate equivalent in energy value to the 



