METABOLISM 191 



stances are not disintegrated or "burned" in the organism, and 

 thus are not sources of energy. There is, of course, more or less 

 interchange of radicles, and to a certain extent inorganic sub- 

 stances are built into compounds of organic nature. But quite 

 aside from this, inorganic materials play a very important part 

 in metabolism. Numerous chemical reactions in the body are 

 controlled, or influenced by salts. The irritability of muscle and 

 nerve is greatly affected by the kind and amount of salts pres- 

 ent. The clotting of blood and of milk both are dependent upon 

 the presence of calcium. The general osmotic equilibrium of 

 tissues and fluids depends in large measure upon salts. Even 

 the development of the unfertilized eggs of some animals may 

 be stimulated by certain salts. Evidently the inorganic 

 materials of the body have far more extended importance than 

 merely to form an inert framework to support and protect the 

 soft organs and tissues. 



Energy Exchange. A very important phase of metabolism 

 is concerned with the body's energy requirements. Very amus- 

 ing ideas on this subject prevailed up to a century and a half 

 ago. The foundations of present day knowledge were laid by 

 the great Frenchman, Lavoisier, who demonstrated that there 

 was something in common between the burning of a candle and 

 the breathing of an animal. Mice died and candles went out if 

 placed under a bell jar, and either one shortened the period of 

 a subsequent mouse or candle. He decided that the burning of 

 the candle consisted in a combining of the carbon of the candle 

 with a substance in the air which he called ' ' oxygen, ' ' and that 

 in animals a similar process took place, producing the heat of 

 the body. By measuring the amount of ice melted in a given 

 time by the heat from an animal, and comparing the result with 

 the amount of carbon dioxide produced by the animal in a period 

 of similar length, he obtained results which indicated that at 

 least 96% of the heat produced by the animal could be accounted 

 for by the oxidation of carbon to C0 2 . He further showed that 

 oxygen also was used up in combining with hydrogen to form 

 water. He also arrived at the result that more heat was pro- 



