176 SPECIAL PHYSIOLOGY 



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 proportions that would be observed in the dialyzer. 

 This need occasion no surprise; in one case we have to deal with 

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



Living cells 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 according to laws which conform not at all to the laws of 

 osmosis. 



In order, however, to understand the current literature on the sub- 

 ject of absorption, it is necessary to be familiar with the terminology 

 and the 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. 



1. Appliances and Materials. Six dialyzers complete, includ- 

 ing outer receptacles and supports, two or three 100 c.c. evaporating 

 dishes, distilled water, sodium chloride, alcohol, egg, and mercury 

 manometer. 



2. Preparation. (1) Fit four of the dialyzers with membrane of 

 pig bladder. The bladders should be carefully selected as to uni- 

 formity in thickness, and should be soaked for an hour or more in 

 water before being stretched upon the dialyzers. The membrane 

 should be stretched as nearly uniform as possible upon four dialyzers. 

 Fit one dialyzer with parchment paper, such as is frequently used 

 for this purpose. Furnish one dialyzer with some other animal mem- 

 brane e.g., a cow's bladder or a rabbit's caecum. 



(2) Prepare dilute egg albumin by adding to strained undiluted 

 albumin about nine volumes of distilled water. 



3. Experiments and Observations. (1) Salt, in saturated aque- 

 ous solution, may be put into a dialyzer. So adjust the apparatus 

 that the water in the outer receptacle shall be on a level with the 

 solution in the vertical tube of the dialyzer. How much does the 

 water rise in the tube? What degree of positive pressure within the 

 dialyzer does that represent? How much pressure per unit area, 

 measured with a mercury manometer will it be necessary to produce 



