22 THE PHYSIOLOGY OF PROTEIN METABOLISM 



about 73 per cent, of the total residual nitrogen. In using the tannic 

 acid precipitation method they found an increase in the non-precipit- 

 able nitrogen in the blood of the digesting animal as compared with 

 the fasting animal. The increase amounted to '007 grm. nitrogen in 

 100 c.c. blood which equals, as they have calculated, -0705 grm. 

 leucine, or much less glycine. They carried out a couple of ex- 

 periments in which they fed an animal with deutero-proteose, but 

 they were unable to detect any increase of the proteose fraction in 

 the serum above the amount present in a meat-fed dog. 



Cathcart and Leathes (92) attempted to demonstrate the increase 

 of the non-precipitable nitrogen (by tannic acid) in the blood following 

 absorption, by performing a perfusion of the isolated intestine in which 

 the same blood was repeatedly used, and in which therefore the products 

 from absorption would accumulate. Owing, however, to the fact that 

 the mucous membrane of the intestine ceased to functionate this method 

 had to be abandoned. By utilizing the whole animal and allowing the 

 absorption of peptone, proteoses or tryptic digestion products to take 

 place from a limited portion of the intestine, a constant increase of the 

 rion-precipitable nitrogen in the blood was found. It was definitely 

 shown that the whole increase was not due to urea and ammonia. In 

 all the experiments control blood was taken from the animal before 

 any digestion products were introduced into the intestine. The am- 

 monia was found to account for about 4 per cent, and urea for 

 nearly 50 per cent, of the non-precipitable nitrogen. By this method 

 only some 1 5 per cent, of the nitrogen, which was absorbed, could be 

 accounted for by the increase of the non-precipitable nitrogen in the 

 blood, and when the liver was also examined about another 1 5 per 

 cent, was found to be retained there. Further an attempt was 

 made to detect the presence of an increase in the coagulable protein 

 (which was to be expected if the immediate synthesis hypothesis were 

 correct), but no such increase could be detected although full allowance 

 was made for the dilution of the blood by fluid absorbed from the in- 

 testine. In carrying out these tests not only was the alteration of the 

 dilution of the blood controlled by estimations of the total nitrogen of 

 the blood, but the haemoglobin content was also estimated. The con- 

 clusion reached was that the blood could not be looked on as a storage 

 place for nitrogenous nutriment, but was concerned simply with the 

 transport of such material. The question of deaminization is one of 

 very great importance in this connexion, because if, as is probably the 

 case, the amino group be rapidly split off from the greater proportion 

 of the absorbed amino acids, a large increase in the non-precipitable 



