ECONOMY OF THE BODY 131 



is a laboratory ; the furnishings of the laboratory are made 

 of proteins ; as the everyday operations go on there is wear 

 and tear of these ; and thus there must arise nitrogenous waste- 

 products. These are poisonous, auto-intoxicant, fatiguing — 

 and they must be got rid of, which is the function of excretion. 



But this is not the whole story. All animals eat protein 

 food ; and very generally, too much of it. The chemical 

 energy of this food-material makes it practicable for the 

 protein- organisation of the animal to make a stir in the 

 world. Proteins mean munitions ; and, although plants 

 make the bulk of them (though not the best of them), plants 

 have hardly learned to use them. 



The proteins of the food are changed by digestive 

 ferments into amino-acids which are distributed by the 

 blood. These serve in small part as material for growth 

 and for the repair of the protein framework in the cells of 

 the body. But in great part the amino-acids are chemically 

 changed to form hydrocarbon acids which are burnt within 

 the body to supply energy. This chemical change takes 

 place mainly in the liver, and it is associated with the forma- 

 tion of ammonia. This goes to form urea in mammals, 

 and this is filtered out by the kidneys. In birds, the result 

 of the kidney's filtration is a clear liquid in the duct (or 

 ureter) which carries it away ; but this duct opens into the 

 cloaca (the dilated end of the food-canal, occurring, for 

 instance, in reptiles, birds, and the three egg-laying 

 mammals), and the filtrate is delayed there. In the bird 

 the walls of the cloaca appear to absorb water from the 

 filtrate, which thus becomes semi-solid, and consists mainly 

 of urates or salts of urea. 



We see then that the nitrogenous waste-products of 

 the bird's body are partly due to the wear and tear of the 

 protein framework of the cells, and partly to the nitrogenous 

 residue of the food-proteins which are burnt away as a 

 source of energy. In the same way, in an engine we might 

 distinguish fine particles of steel, due to the actual wear and 

 tear of the machinery, from the ashes which resulted from 

 unused fragments of the fuel. 



