A STUDY OF BLOOD MANUFACTURE 103 



raw white of egg mixed with water. At the end of several 

 days the liquids in both thistle tubes will be found some- 

 what higher than the level of the water outside, showing 

 that the denser starch paste and proteid mixtures have 

 absorbed some water. But when we test the water outside 

 the starch by adding iodine, we fail to find a trace of starch ; 

 and on adding nitric acid and ammonia to the water in the 

 second experiment, we demonstrate that little or none of the 

 proteid has passed through the intestine. 



We have found, then, that some substances pass readily 

 through an animal membrane while others do not. To the 

 former class is given the name crys'tal-loids because many of 

 these substances have a crystalline form. As examples of 

 crystalloids we may mention sugar, salt, water (crystalline 

 when frozen), and peptones (without crystalline form). 

 Col'loids (Greek = gluelike), on the other hand, include 

 proteids, starch, and gum, which do not readily soak through 

 an inclosing membrane. 



Application of the Principles of Osmosis to Absorption. We 

 can now readily see the necessity for the change of starch 

 into sugar, and of proteids into peptones, and for the emul- 

 sion or saponification of fats. Unless these nutrients are 

 converted from colloids into crystalloids, they cannot pass 

 through the membranes that separate them from the blood. 

 Although absorption cannot be wholly explained by applying 

 the principles of osmosis, yet this is undoubtedly one of the 

 most important factors in the process. 



The most favorable conditions for rapid absorption are 

 these : (1) a considerable extent of moist, absorbing mem- 

 brane, (2) a rich supply of blood and lymph vessels beneath 

 this membrane, and (3) the digested food must remain for 

 some time in close contact with the absorbing surface. 

 Keeping in mind these requirements, let us consider the 

 opportunities for absorption offered in each region of the 

 alimentary canal. 



Absorption in the Mouth, Throat, and Gullet. While the 



