520 THE ABSORPTION OF FOOD [CH. XXXIV. 



passes. Cano sugar and milk sugar are also converted into glucoses 

 before absorption. 



The carbohydrate food which enters the blood as glucose is taken 

 to the liver, and there stored up in the form of glycogen a reserve 

 store of carbohydrate material for the future needs of the body. 

 Glycogen, however, is found in animals who take no carbohydrate 

 food. It must, then, be formed by the protoplasmic activity of the 

 liver cells from their proteid constituents. The glycogenic function 

 of the liver is discussed in the chapter preceding this. Glucose is the 

 only sugar from which the liver is capable of forming glycogen. If 

 other carbohydrates like cane sugar or lactose are injected into the 

 blood-stream direct, they are unaltered by the liver, and finally leave 

 the body by the urine. 



Absorption of Proteids. A certain amount of soluble proteid is 

 absorbed unchanged. Thus, after taking a large number of eggs, egg 

 albumin is found in the urine. Patients fed per rectum derive 

 nourishment from proteid food, though proteolytic ferments are not 

 present in this part of the intestine. 



Most proteid, however, is normally absorbed as peptone and 

 proteose or their decomposition products. Peptones and proteoses 

 are absent from the blood under all circumstances, even from the 

 portal blood during the most active digestion. In other words, during 

 absorption the epithelial cells change the products of proteolysis back 

 once more into native proteids (albumin and globulin). 



The greater part of the proteid absorbed passes into the blood; 

 a little into the lymph also; but this undergoes the same change. 



When peptone (using the word to include the proteoses also) is 

 injected into the blood-stream, poisonous effects are produced, the 

 coagulability of the blood is lessened, the blood-pressure falls, secre- 

 tion ceases, and in the dog 0'3 gramme of "peptone" per kilogramme 

 of body-weight is sufficient to kill the animal. 



The epithelial cells of the alimentary canal thus protect us from 

 those poisonous effects by converting the harmful peptone into the 

 useful albumin. 



The whole question of proteid absorption is in a very unsettled state just now. 

 Cohnheim's discovery of erepsin (p. 497) appears to lend support to the view that 

 the peptones are very largely broken up into simpler substances, but the absence 

 of these in the blood-stream shows that the absorptive epithelium is capable of 

 resynthesising them into proteids. Several observers have noted the small amount 

 of peptones obtainable from the intestinal contents ; this may be due to the fact 

 that they are so rapidly absorbed, or it may be due to their having been broken up 

 into simpler substances by trypsin and erepsin. On the other hand, there 

 are some observers who hold that the importance of erepsin has been exag- 

 gerated, and that tli3 absorptive epithelium can also resynthesise proteids from 

 proteoses and peptone. It is, however, undeniable that the body can be maintained 

 in health and nitrogenous equilibrium by feeding it on the final crystalline products 

 of pancreatic action (Loewi), and it is probable that the synthesis of the body 

 proteids from these is accomplished mainly by the intestinal epithelium, 



