FATE OF THE FOOD ABSORBED 391 



is difficult, because, when the portal vein is ligatured, the blood 

 returning to the heart tends to accumulate in the great veins 

 of the abdomen. But this difficulty has been overcome by 

 Eck, who devised a method of connecting the portal vein with 

 the inferior vena cava, and finally occluding the portal vein, 

 and of thus allowing the blood to return from the abdomen 

 to the heart. 



Source of Urea,. Urea is produced from the decomposition 

 products of proteins of the food and tissues (see p. 216). The 

 manner in which excess of protein in the food is broken down 

 into ammonia compounds in the intestine and sent to the liver 

 has been already considered (p. 380). But the fact that even in 

 starvation urea is produced seems to indicate that the initial 

 stages of decomposition of proteins may go on elsewhere than 

 in the intestinal wall. The fate of haemoglobin tends to show 

 that the whole process may be conducted in the liver cells. 

 When haemoglobin is set free from the corpuscles, the nitrogen 

 of its protein part is changed to urea, while the pigment part 

 is deprived of its iron and excreted as bilirubin. Whether 

 the proteins of muscle and other tissues are thus directly dealt 

 with, or whether the initial stages of decomposition go on 

 outside the liver, is not known. But the wide distribution 

 of erepsin through the tissues may indicate that the initial 

 splitting of the protein goes on in them. 



The nitrogen excreted is not all in the form of urea, but 

 some is combined in ammonia salts, in uric acid and other 

 purin bodies (see p.' 427), and in creatinin. In the mammalian 

 body ammonia and the purin bodies can be changed into urea, 

 and it is probable that the small amounts of these substances 

 which appear in the urine have simply escaped this conversion. 

 Certain drugs (alcohol, sulphonal, etc.) and toxins (diphtheria) 

 markedly decrease their conversion into urea and so increase 

 their quantity in the urine. Although urea may be prepared 

 from creatin, there is no evidence that the process goes on 

 in the body. Creatin yields the creatinin of the urine. 



It is probable that after the nitrogenous portion of the 

 protein molecule is split off and got rid of, the liver has the 

 further power of turning the non-nitrogenous part into sugar 

 and either sending it to the tissues or storing it as glycogen. 



Summary of the Functions of Liver. The functions of the 



