74 THE AMINO ACIDS 



per hour is compared to the volume of blood in which 

 it would necessarily be dissolved we find that the 

 protein would be present in the concentration of 0.12 

 per cent. Another difficulty arises from the fact that 

 the blood is a fluid already containing about 3 per 

 cent of coagulable protein and also nitrogen to a 

 smaller extent in other forms. 



Certain fairly definite indications, however, have led 

 many physiologists to maintain a belief that protein 

 is absorbed and is taken up by the cells in the form of 

 amino acids. 



Determination of the "residual nitrogen" of the 

 blood, that is, the difference between the total nitrogen 

 and that representing coagulable protein has shown 

 that after meals there is a slight gain both in the portal 

 blood and that of the systematic circulation. This 

 increase of nitrogen may be attributed to the absorbed 

 amino acids or polypeptides, but in view of the possi- 

 ble existence of non-coagulable proteins in the blood 

 it cannot be accepted as proof positive. A second 

 method for the same endeavor has been the formation 

 of amino acid compounds by use of j3-naphthalene 

 sulphochloride. Shaken with the fluid obtained after 

 separation of the serum proteins either by coagulation 

 or by means of dialysis precipitates have been obtained 

 with this reagent, strongly indicating the presence of 

 amino acids but the failure of the precipitate to assume 

 a crystalline form has made impossible a positive 

 identification of amino acids. Cohnheim by observa- 

 tions with the isolated intestine of the octopus was able 



