232 LECTURE XI. 



however, be kept alive for a considerable time, if the liver is cut off from 

 the general circulation while the portal vein is sewed to the Vena cava 

 inferior near the hilus of the liver, and communication then established 

 between the two veins. The blood then passes directly from the intestine 

 into the general circulation. The experimental results obtained after 

 this operation are not always uniform, owing to the fact that some of 

 the blood will often find its way through the liver on account of a collateral 

 circulation which may develop. 



If, thus, the utilization of ammonia in the formation of urea has been 

 established, we must now determine whether it is to be assumed that all, 

 or at least the greater part, of the amino groups present in the tissues are 

 split off as ammonia, and thus take part in the production of urea. Such 

 an assumption has much in its favor. In recent years a number of pro- 

 cesses have been discovered in the animal organism which indicate the 

 presence of ferments which cause the removal of the amino group, and, 

 in fact, such processes are known in the vegetable as well as in the animal 

 kingdom. We can imagine that entirely analogous to the breaking down 

 of carbohydrates and fats in the tissues, the proteins are first hydrolysed 

 with the formation of the separate amino acids, from which ammonia is 

 split off. Nitrogen-free carbon chains would then remain, by the com- 

 bustion of which the cells could then obtain their energy. Unfortunately, 

 we know nothing further about these carbon chains. It is possible that 

 they enter into relations with the carbohydrates and with the fats. The 

 CO group for the production of urea does not necessarily have to orig- 

 inate from the albumin itself. 



We have already called attention to the assumption that carbamic acid 

 may occur as an intermediate product between the amino acids and urea. 



X NH 2 



It is not known as the free acid: CO , but as its ammonium salt. 



X OH 



X NH 2 X NH 2 



The close relations between carbamic acid, CO , and urea, CO , are 



^OH X NH 2 



evident without further comment. We may consider urea as the amide 

 of carbamic acid. It is difficult to decide, from the investigations at hand, 

 whether this acid is a normal metabolic product. There are many obser- 

 vations at hand which indicate that carbamic acid is present in urine, in 

 fact, normally so. Abel * states that he has succeeded in obtaining this 



1 E. Drechsel and J. J. Abel: Arch. Anat. Physiol. 1891, 236. J. J. Abel and A. 

 Muirhead: Arch. exp. Path. Pharm. 31, 15 (1893). Cf. also E. Drechsel: J. pract. 

 Chem. 12, 417 (1875); 22, 476 (1880). 



