656 LECTURE XXVIII. 



ceivable that the albumin of one kind of milk may be differently constituted 

 from that of another in a quite specific way. In the case of sucklings the 

 most important function of the food is to build up the cells. Within a 

 short time the animal doubles its original weight. We can imagine that 

 some kinds of protein are not suitable for being introduced into the cell. 

 Such ideas were very well justified at the time when it was assumed that 

 the protein was decomposed in the intestine only to the peptone stage and 

 that these products were absorbed, and when there was no evidence at 

 hand concerning the composition of the different proteins of the body. 

 After it was ascertained, however, that the suckling was able from the 

 protein in its food to construct all the different proteins contained in the 

 various fluids of the body and in the tissues, the composition of which is 

 quite different from that of casein, 1 it hardly seemed right for us to lay too 

 much stress upon the quantitative composition of the protein in the food. 

 Even casein is decomposed while it is in the alimentary canal. Outside 

 the intestine the various cleavage-products unite in various ways to form 

 new proteins. Even the albumin contained in milk is made capable of 

 absorption by means of changes which take place while it is in the intes- 

 tine. This does not imply by any means that the chemical composition 

 of the various proteins is entirely a matter of indifference. There are 

 no grounds for any such assumption. It is also conceivable that the 

 caseins from different varieties of milk contain certain specific groups. At 

 present, to be sure, we do not know of any such. All that we do know is 

 that up to the present time the different kinds of casein which have been 

 studied all contain the same building-stones and apparently in about the 

 same quantitative relations. 2 We would, however, far exceed the present 

 state of our knowledge if we were to conclude definitely that all the different 

 varieties are identical because they are composed of the same constituents. 

 It is perfectly clear that the same amino acids may be combined in a num- 

 ber of different ways in the complex molecule. The number of possible 

 isomers is very large. We have already seen in studying fermentations 

 that very slight differences in the construction of the molecule are of much 

 biological significance. 



The casein of human milk differs from that of cow's milk in that rennin 

 throws it down in the form of a much finer flock. It is also easier to pre- 

 cipitate casein from cow's milk by slightly acidifying it with acetic acid. 

 From human milk it is very difficult to precipitate the casein by means of 

 acetic acid. At ordinary temperatures the precipitation is at best very 

 incomplete, and in most cases no precipitate at all is formed. In order to 

 throw down completely the casein from human milk, it is necessary to 

 carefully acidify it slightly, dilute it with water, and keep it at 37 C. for 



* Cf. Lecture X, p. 211. 



2 Emil Abderhalden and Alfred Schittenhelm : Z. physiol. Chem. 47 (1906). 



