WHERE NUTIilEKTS ARE ABSORBED 139 



stance like grape sugar we should expect to find it in 

 blood. The fact remains, however, that while we know 

 that peptone leaves the alimentary tract as peptone it 

 is not only never found in the Mood as peptone, but when 

 injected into the blood it acts as a poison. Now, since 

 the only thing that intervenes between the alimentary 

 tract and the blood and lymph are the cells which com- 

 pose the membranes (mucous membrane and capillary 

 wall cells), it must follow that in passing through these 

 cells the protoplasm in them acts upon the peptone and 

 converts it into another and harmless form of proteid 

 and delivers it in this form to the blood. What the exact 

 nature of this protoplasmic action is, is still unknown. We 

 may, however, sum up the facts known as to transfer of 

 foods as follows: 



Summary. Digestion by transforming foods to crystal- 

 loids makes possible their transfer into the blood and 

 lymph. This passage may or may not be accompanied 

 by changes in the character of the substances in solution, 

 according as the protoplasm does or does not act upon 

 them in transit. Before leaving this subject, however, it 

 must be stated that since the membranes concerned are 

 made up of separate cells, there may be some seeping 

 of liquid through the spaces between the cells. Such pas- 

 sage of liquids is called filtration, and is not to be con- 

 fused with osmosis. Since these spaces are exceedingly 

 minute, it is safe to say that the osmotic exchange 

 accounts for the transfer of the major part of the food. 



Where nutrients are absorbed. — Proteid. While some 

 of the proteid is changed to peptone in the stomach, 

 the amount of peptone absorbed by the blood vessels of 

 the stomach is relatively small, as most of the peptone is 



