31(3 AN AMERICAN TEXT- HOOK OE PHYSIOLOGY. 



Opinion- as to why these forms of proteids are more easily absorbed than any 



other must vary with the theory held as to the nature of absorption. Ex- 

 periments have shown thai proteoses and peptones are more easily diffusible 

 than other forms of proteids, and this Pact tends to support the view that 

 their absorption is due to physical diffusion. The object of digestion, on this 

 view, is to convert the insoluble and non-dialyzable proteids into soluble, 

 diffusible peptones. But a study of the details of proteid absorption has 

 shown that the process cannot be explained entirely by the laws of simple 

 dialysis that govern the process <>f diffusion through dead membranes. Pro- 

 teids, like egg-albumin, which are practically non-dialyzable are absorbed 

 readily from the intestine. Moreover, when one considers the rate of absorp- 

 tion of peptone from the alimentary tract, it seems to be much too rapid and 

 complete to be accounted for entirely by the diffusibility of this substance as 

 determined by experiments with parchment dialyzers. It is believed, there- 

 fore, that the initial act in the absorption of proteids is dependent in some 

 way upon the peculiar properties of the layer of living epithelial cells lining 

 the mucous membrane. Whether the peculiarity is a physical one depending 

 on some special structure of the cells that makes them permeable to the pro- 

 teid molecules, or whether it is a more obscure and complicated process con- 

 nected with the living activity of the cells, remains undetermined for the 

 present. After the proteids have passed through the epithelium it is a 

 matter of importance to determine whether they enter the blood or the 

 lymph circulation. Experiments have shown conclusively that they are 

 transmitted directly to the blood-capillaries: ligature of the thoracic duct, 

 for example, which shuts oil' the entire lymph-How coming from the intes- 

 tine, does not interfere with the absorption of proteids. There is one other 

 fact of great significance in connection with this subject: the proteids are 

 absorbed mainly, if not entirely, as proteoses and peptones, and they pass 

 immediately into the blood ; nevertheless, examination of the blood directly 

 after eating, while the process of absorption is in full activity, fails to show 

 any peptones or proteoses in the blood. In fact, if these substances are 

 injected directly into the blood, they behave as foreign, and even as toxic, 

 bodies. In certain doses they produce insensibility with lowered blood- 

 pressure, and they may bring on a condition of coma ending in death. 

 Moreover, when present in the blood, even in small quantities, they are 

 eliminated by the kidneys and are evidently unfit for the use of the tissues. 

 It follows from these facts that while the peptones and proteoses arc being 

 absorbed by the epithelial cells they arc at the same time changed into some 

 other form of proteid. YVhal this change is has not been determined. Ex- 

 periments have Bhown thai peptones disappear when brought into contact 

 with fresh pieces of the lining mucous membrane of the intestine which are 

 -till in a living condition. The statement has been made that the peptones 

 and proteoses are converted to serum-albumin, or at least to a native albumin 

 of some kind, but we have no definite knowledge beyond the fact that the 

 peptones and proteoses, a- such, disappear. It is well to call attention to the 



