DIGESTION. 369 



probable that but little of the proteid is broken up beyond 

 the peptone stage, and all of it never is; an album ose is 

 formed as an intermediate product. The enzyme concerned is 

 trypsin ; it is active only in an alkaline or neutral medium, 

 and before dissolving solid proteids does not cause them to 

 swell and become transparent as pepsin does. Like the other 

 digestive ferments, it is most active at about the temperature 

 of the Body, and is destroyed by boiling. On fats the pan- 

 creatic secretion has a double action. To a certain extent it 

 breaks them up, with hyd ration, into free fatty acids and 

 glycerin; for example 



(C 'C H H 0) ' 1 3 + 3H ' = 3 ( 18Hs H 1 ) + 3 H 6 



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



The fatty acid then combines with some of the alkali present 

 to make a soap, which being soluble in water is capable of 

 absorption. Glycerin, also, is soluble in water and dialyz- 

 able. The greater part of the fats are not, however, so broken 

 up, but are simply mechanically separated into 

 which remain suspended in the chyle and give it 

 color, just as the cream -drops are suspended in milk, or the 

 olive-oil in mayonnaise sauce. This is effected by the help of 

 a quantity of albumin which exists dissolved in the pancreatic 

 secretion. In the stomach, the animal its eaten have lost 

 their cell-walls, and have become melted by the temperature 

 to which they were exposed. Hence their oily part floats free 

 in the chyme when it enters the duodenum. If oil be shaken 

 up with water, the two cannot be got to mix; immediately 

 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 menstruem. The reason of this is that each oil- 

 droplet becomes surrounded by a delicate pellicle of albumin, 

 and is thus prevented from fusing with its neighbors to make 

 large drops, which would soon float to the top. Such a mix- 

 ture is called an emulsion, and the albumin of the pancreatic 

 secretion emulsifies the oils in the chyle, which becomes 

 , white (for the same reason as milk is that color) because the 

 innumerable tiny oil-drops floating in it reflect all the light 

 which falls on its surface. 



In brief, the pancreatic secretion converts starch into 



