370 HUMAN PHYSIOLOGY 



mammals is difficult, because, when the portal vein is liga- 

 tured, 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 uniting the 

 peripheral end of the divided portal vein with the inferior 

 vena cava, and of thus allowing the blood to return from the 

 abdomen to the heart. 



Source of Urea. Urea is produced from the proteids of 

 the food and tissues. The manner in which excess of proteid 

 in the food is broken down into ammonia compounds and 

 sent to the liver has been already described (p. 369). But 

 the fact that even in starvation urea is produced, seems to 

 indicate that the initial stages of decomposition of proteids 

 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 proteid part is 

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

 iron and excreted as bilirubin. Whether the proteids 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. It is probable that lactate of ammonia 

 is produced in muscle, and that this is converted into urea 

 in the liver. 



Speculations as to the way in which the proteid molecule 

 is broken down are of little value. It is one thing to show 

 that urea may be formed in a particular way outside the 

 body, but quite another to prove that it is formed in that 

 particular way in living protoplasm. 



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. 397), and in creatinin. In the mam- 

 malian 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, &c.) 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 

 such a process goes on in the body. 



