XLIII. Absorption. 



Physiologists have entertained the hope that all the 

 phenomena of absorption of diffusible substances could 

 be eventually explained by the laws of physics. That 

 hope has practically given place to the conviction that 

 however important it may be to the animal economy to 

 produce, in its digestive processes, diffusible products, 

 these products do not pass through the epithelial lining 

 of the alimentary tract at the rate or in the propoitions 

 that would be observed in the dialyzer. This need oc- 

 casion no surprise; in one case we have to deal with 

 living, active cells, in the other with dead tissue. 



Living ceils of muscle-tissue or of gland-tissue have the 

 power of selecting from the tissue plasma such materials as 

 are needed for the replenishment of their substance. Not 

 only does the animal select what shall be taken into the 

 alimentary tract but the epithelial lining of that tract 

 seems to select what shall be absorbed and to absorb it ac- 

 cording to laws which conform only in a most general way 

 or which may not conform at all to the laws of osmosis. 

 In order, however, to understand the current literature on 

 the subject of absorption it is necessary to be familiar with 

 the terminology and laws of osmosis and dialysis. To 

 that end the student may profitably perform for himself a 

 few simple experiments preliminary to more complex ones 

 which the demonstrator may suggest or may perform for 

 the class. 



/. Appliances and Materials. Six dialyzers complete, in- 

 cluding outer receptacles and supports; 2 or 3, 100 c. c., 



189 



