ABSORPTION AND ASSIMILATION OF FOODSTUFFS 143 



are injected into the blood, they are rapidly excreted by the 

 kidneys. 



The change from proteoses to simple proteids takes place 

 in the epithelial cells of the intestine, for the native proteids 

 of the body are increased when no other proteids than pro- 

 teoses are given in the food. Hence the real body proteid 

 must have been formed from the proteoses of the food. 



If a solution of proteoses is heated at body temperature with 

 the fresh mucous lining of the intestine, the proteoses gradually 

 disappear without the further formation of decomposition products. 

 This supports the view that the proteoses are changed back to 

 native proteids. 



Solutions of native proteids, acid and alkali albumin can 

 also be absorbed as such without previous digestion. It has 

 been observed that such proteids, placed in an isolated loop 

 of intestine free from ferment, are rapidly and completely 

 absorbed without albumoses and peptones having been 

 formed. These proteids, injected into the blood are 

 assimilated and used by the body ; this, however, is not true 

 of caseinogen, egg albumin, and haemoglobin, for they are 

 rapidly excreted by the kidneys if injected into the blood. 



Large quantities of egg albumin solution taken into the 

 stomach can also be absorbed without previous digestion, 

 but are then excreted by the kidneys. Caseinogen and 

 haemoglobin are precipitated in the stomach and are there- 

 fore not absorbed without digestion and the transformation 

 into new proteid in the intestinal wall. This perhaps ex- 

 plains the meaning of the caseinogen coagulation by rennin, 

 for, if the caseinogen was not precipitated, it might be 

 absorbed as such and then be excreted by the kidneys. 



Many soluble proteids can, therefore, be absorbed and 

 assimilated and used in the body without being digested or 

 changed ; others, on the contrary, must first be digested. 

 The object of digestion is therefore, first, to render insoluble 

 proteids soluble and, secondly, to change the proteids which 

 are soluble but cannot be assimilated to proteids which can 

 be assimilated. 



