CHEMISTRY OF DIGESTION AND NUTRITION. :U7 



fact that the digestion of proteids is supposed, according to the schema already 

 described, to consist in a process of hydration and splitting, with the forma- 

 tion, probably, of smaller molecules. The reverse act of conversion of pep- 

 tones hack to albumin implies, therefore, a process of dehydration and poly- 

 merization that presumably takes place in the epithelial cells. It is at this 

 point in the act of absorption of proteids that our knowledge ismosl 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 sugar-splitting 

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

 sugar is hydrolyzed into dextrose and levulose before absorption, and milk- 

 sugar possibly undergoes a similar change to 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 in- 

 jected 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 intro- 

 duced 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 

 cn*r of peptones, exact study of their absorption shows that it does not fellow 

 in detail the known laws of osmosis through dead membranes. Experiments 

 indicate, however, that in a general way the behavior of solutions of sugar 

 placed in isolated loops of the intestine may be understood by assuming that 

 a diffusion takes place, and it may be therefore that the peculiarities observed 

 are connected with the structure of the living epithelium. We have to deal 

 here, in fact, with the same difficulty as was encountered in the case of the 

 proteids. A special vital activity of the epithelial cells ciinnot be excluded, 

 and we must be content to await a fuller development of experimental inves- 

 tigation before attempting to come to a final conclusion. As in the case of 

 the proteids, the absorbed sugars — dextrose or dextrose 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 1 ), a distinct increase in its 

 sugar-contents being found. Examination of the lvmph shows no increase in 

 sugar unless excessive amounts of carbohydrates have been eaten (Ileiden- 

 hain). 



Absorption of Fats. — As has been stated, fats are absorbed either in 

 solid form, as emulsified droplets, or as fatty acids or soaps. In the latter 

 case the fatty acids are again recombined tit particles of neutral fat, pre- 

 sumably within the substance of the epithelial cells. So far as the emulsified fat 

 1 Dn Bois-Reymond's Archivfur Analomit uml Physiologie, 1^77, S. II.",. 



