256 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



are more easily diffusible than are other forms of proteids harmonized with 

 this theory. The object of digestion, it was said, 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 by the laws of simple dialysis which govern the process of diffusion 

 through dead membranes. Proteids, like egg-albumin, which are practically 

 non-dialyzable are absorbed readily from the intestine. Moreover, when one 

 considers the rate of absorption of peptone from the alimentary tract, it seems 

 to be much too rapid and complete to be accounted for entirely by the dif- 

 fusibility of this substance as determined by experiments with parchment 

 dialyzers. It is believed, therefore, that the initial act in the absorption 

 of proteids is dependent in some way upon the properties of the living 

 epithelial cells lining the mucous membrane. It is impossible at present 

 to make this statement more specific. A second similar suggestion attributes 

 the absorption of proteids to the leucocytes found so abundantly in the 

 adenoid tissue of the intestine, but this has been shown by Heidenhain l 

 and others to be incorrect. We say, then, in brief, that the peptones and 

 proteoses are absorbed by a special activity of the epithelial cells. Are they 

 then transferred to the blood or to the lymph? Experiments have shown 

 conclusively that they are transmitted directly to the blood-capillaries : liga- 

 ture of the thoracic duct, for example, which shuts off the entire lymph-flow 

 coming from the intestine, does not interfere with the absorption of proteids. 

 There is one other fact of great significance in connection with this sub- 

 ject: 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 are being 

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

 other form of proteid. What this change is has not been determined. 

 Experiments have shown that peptones disappear when brought into contact 

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

 still in a living condition. The presumption is 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, as such, disappear. It is well to call attention to the 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 formation, probably, 

 of smaller molecules. The reverse act of conversion of peptones back to albu- 

 1 Pftiiger's Archivfilr die yesammte Physiologic, vol. 43, 1888, supplement. 



