160 THE HUMAN BODY. 



different substance, trypsin, which differs from pepsin in 

 acting in an alkaline instead of in an acid medium. On 

 fats it has a double action. To a certain extent it breaks 

 them up into fatty acids and glycerine.* The fatty acid 

 then combines with some of the alkali present to make 

 a soap, which being soluble in water is capable of absorp- 

 tion. f Glycerine also is soluble in water and capable 

 of absorption. The greater part of the fats is not, how- 

 ever, so broken up, but simply mechanically separated 

 into little droplets which remain suspended in the chyle 

 and give it a whitish color; just as cream-drops are sus- 

 pended in milk, or olive oil in mayonnaise sauce. If oil bo 

 shaken up with water, the t\vo cannot be got to mix; immedi- 

 ately the shaking ceases the oil floats up to the top; but if 

 some raw egg be added a creamy mixture is readily formed 

 in which the oil remains for a long time evenly suspended 

 in the watery menstruum. The reason of this is that each 

 oil-droplet becomes surrounded by a delicate pellicle of 

 albumen, and is thus prevented from fusing with its neigh- 

 bors to make large drops which would soon float to the top. 

 Such a mixture is called an emulsion, and the albumen of 

 the pancreatic secretion emulsifies the oils in the chyle, 

 which becomes white (for the same reason as milk is that 



How does trypsin differ from pepsin ? How does pancreatic secre- 

 tion break up some fats ? What digestive end is thus attained ? 

 How is most of the fat eaten acted upon by the pancreatic secretion ? 

 Why is the chyle white ? How may we mix oil with water ? Ex- 

 plain the process. What is an emulsion ? What emulsifies the oily 

 matters of the chyle ? 



* 



= 3 



1 Stearin + 3 Water = 3 Stearic acid -f 1 Glycerine. 

 t Ordinary soap is a compound of a fatty acid with soda, colored and scented 

 by the addition of various substances. Soft soap is a compound of a fat|; er '-' 

 with potash. Both dissolve in water, which the fats from which thev.^" 

 willi.otdo. 



f! 



