THE HISTORY OF UREA 189 



know as yet exactly in what form it does leave them. 

 In fact all we know is that the muscles give off nitrogenous 

 waste, that this waste is presumably turned into urea 

 in some other part of the body, and the urea picked out 

 and excreted by the kidneys. 



But there is another organ in the body of great size 

 and importance, the liver (p. 207). This organ is the 

 seat of many activities with which we shall deal later on, 

 among which the making, of urea out of other substances 

 brought in the blood is not the least important. We 

 also know to a certain extent what these " other sub- 

 stances " are. "When we study digestion we shall see 

 that the products of digestion of proteins are nitro- 

 genous, crystalUne substances known as amino-acids. 

 These are absorbed through the walls of the intestines, 

 carried to the liver in the blood of the portal vein, 

 and in part converted into urea hy the liver. Moreover 

 there are instances of animals which have survived 

 the functional removal of the liver for several days. 

 In such cases the excretion of urea by the kidney is 

 suspended and at the same time there is an accumulation 

 of ammonia in the blood. As it is known that the liver 

 has the power of converting ammonia into urea it seems 

 clear that a considerable proportion of the urea excreted 

 has ammonia as an antecedent. Whatever its exact 

 antecedents we may regard the urea secreted as coming 

 from two main sources, the waste of the tissues and the 

 superfluous nitrogen of the food which is undergoing 

 digestion in the alimentary canal ; the urea derived from 

 tissues is called endogenous uvea,, that derived directly from 

 the food, the antecedents of which have never, therefore, 

 been built into the living substance of the body, is called 

 exogenous urea. In both cases the final stage in the form- 

 ation of urea may be regarded as taking place in the liver. 

 7. The Structure of the Skin. Nails and Hairs.— 

 That the skin is a source of continual loss to the blood 

 may be proved in various ways. If the whole body of a 

 man, or one of his limbs, be enclosed in a caoutchouc bag, 



