14 THE PHYSIOLOGY OF PROTEIN METABOLISM 



also carried out a few experiments in which they injected a dog fed on 

 a carbohydrate diet (potatoes) and found that now a well-marked re- 

 tention of nitrogen took place without any formation of precipitin. 

 They continued their investigations (148) and found that the condition 

 of the nutrition of the dogs and goat played a very important part in 

 this utilization of protein. It was absolutely immaterial whether the 

 injected protein was the animal's own serum, or foreign serum, or egg 

 albumin, since the result was always the same, namely that if the 

 material were injected parenterally there was an increase in the break- 

 down of protein with a rise in the nitrogen excretion above the amount 

 injected. When, however, the animal was fed on a carbohydrate diet 

 no increase was found, indeed a retention of nitrogen was observed. 

 This retention was so great that they were able to keep an animal in 

 nitrogenous equilibrium by protein administered parenterally. They 

 could not prove, however, that any addition of actual body substance 

 occurred. They came to the general conclusion that parenterally in- 

 troduced protein would be broken down and utilized by the body, but 

 that the precipitin reaction was of no value for these investigations, since 

 although any excess of the injected protein might be eliminated in a 

 day or two, the precipitin reaction persisted for a much longer period. 

 Mendel and Rockwood (279) have shown that edestin and excelsin, 

 when introduced into the circulation, can apparently be retained in the 

 organism, for they are not eliminated unchanged in the urine; when 

 introduced into the peritoneal cavity they were also found to disappear. 

 Borchardt (67), like Giirber and Hallauer(i6i), found that the urine was 

 not the only channel of excretion for parenterally introduced proteins ; 

 the non-appearance in the urine of the substance injected could not 

 therefore be accepted as absolute evidence of utilization. This worker 

 injected intravenously hemielastin ; a part of it was found in the wall 

 of the small intestine. He concluded, therefore, that this material was 

 either on its way to the intestine for excretion or was present to under- 

 go certain changes which would render it suitable for utilization by 

 the body, or that it was in the process of absorption after excretion 

 by way of the bile. This last hypothesis was not considered likely, 

 as no trace of hemielastin was discoverable in the liver. Michaelis 

 and Rona (282) made an attempt to replace part of the nitrogen in 

 the diet (of meat, milk, etc.) by injecting an equivalent amount of 

 caseinogen subcutaneously into an animal in nitrogenous equilibrium. 

 They found, however, as Friedemann and Isaac had previously found, 

 that there was an increase in the output of nitrogen in the urine. The 

 caseinogen, however, was not excreted in the urine as such, and like 



