146 PRINCIPLES OF PHYSIOLOGY 



which the liver is supplied by the portal 

 system contains during the digestive and 

 absorptive processes large amounts of pro- 

 teid and of carbo-hydrate in the form of 

 grape sugar. What happens to the proteid 

 has already been discussed (p. 66). A portion 

 of it, however, may be decomposed into a 

 nitrogenous and a non-nitrogenous portion. 

 The nitrogenous portion may, by synthesis, 

 assist in the formation of other molecules of 

 proteid or of certain bodies found in the liver, 

 such as the complicated bile acids which, 

 united to soda to form the bile salts, appear in 

 the bile (p. 140). In the chemical changes 

 affecting the nitrogenous portion one of the 

 bodies formed is undoubtedly urea (p. 132). 

 This substance often represents an excess of 

 proteid in the diet ; at all events a meal rich 

 in proteid is followed by an increased forma- 

 tion of urea. The urea is washed out of the 

 liver by the blood, but as it is quickly removed 

 by the kidneys, it appears in the urine, and 

 the percentage amount in the blood is always 

 small. On the other hand, the non-nitro- 

 genous residue of the proteid may apparently 



