CHEMISTRY OF DIGESTION AND NUTRITION. 257 



min implies, therefore, a process of dehydration and polymerization which 

 presumably takes place in the epithelial cells. It is at this point in the act 

 of absorption of proteids that our knowledge is most deficient. 



Absorption of Sugars. The carbohydrates are absorbed mainly in the 

 form of sugar or of sugar and dextrin. Starches are converted in the intes- 

 tine into maltose or maltose and dextrin, and then by the inverting enzymes 

 of the mucous membrane are changed to dextrose. Ordinary cane-sugar 

 suffers inversion into dextrose and levulose before absorption, and milk-sugar 

 possibly undergoes a similar inversion into dextrose and galactose, though less 

 is known of this. So far as our knowledge goes, then, we may say that the 

 carbohydrates of our food are eventually absorbed in the form mainly of 

 dextrose or of dextrose and levulose, leaving out of consideration, of course, 

 the small part that normally undergoes bacterial fermentation. In accordance 

 with this statement, we find that the sugar of the blood exists in the form 

 of dextrose. It is apparently a form of sugar that can be oxidized very 

 readily by the tissues. In fact, it has been shown that if cane-sugar is injected 

 directly into the blood, it cannot be utilized, at least not readily, by the tissues, 

 since it is eliminated in the urine ; whereas if dextrose is introduced directly 

 into the circulation, it is all consumed, provided it is not injected too rapidly. 

 The sugars are soluble and dialyzable, but, as in the case of peptones, exact 

 study of their absorption shows that it does not follow the known laws of 

 osmosis. The degree of absorption of the different sugars does not vary directly 

 with their diffusibility. Moreover in the small intestine at least the rate of 

 absorption increases with the concentration of the solution only up to a certain 

 point (with dextrose, 5 to 6 per cent.) at which the maximum of absorption 

 takes place, whereas, if it were simply a case of osmosis, the rapidity of dif- 

 fusion ought to increase with an increase in concentration of the solution on 

 one side of the membrane. For these and for other reasons it seems that the 

 absorption of sugars is also a special act depending, in all probability, upon 

 the living epithelial cells. Their absorption seems to be effected by means 

 similar to those used for the proteids, but the details of the act cannot be 

 given. As in the case of the proteids, the absorbed sugars dextrose or dex- 

 trose and levulose pass directly into the blood, and do not under normal 

 conditions enter the lymph-vessels. This has been demonstrated by direct 

 examination of the blood of the portal vein during digestion (Von Mering *), 

 a distinct increase in its sugar-contents being found. Examination of the 

 lymph shows no increase in sugar unless excessive amounts of carbohydrates 

 have been eaten (Heidenhain). 



Absorption of Fats. Unlike the sugars and peptones, fats are absorbed 

 chiefly in a solid form that is, in an emulsified condition. There can be no 

 question therefore, in this case, of osmosis ; the process of absorption must be 

 of a mechanical nature. The details of the process have been worked out 

 microscopically and have given rise to numerous researches. It is unnecessary 

 to speak of the various theories that have been held, as it has been shown by 



1 Du Bois-Keymond's Archivfur Anatomic und Physiologic, 1877, p. 413. 

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