The Pancreatic Secretion is clear, watery, alkaline, and much like saliva in appearance. The Germans call the pancreas the "abdominal salivary gland." In digestive properties, however, the pancreatic secretion is far more important than the saliva, acting hot only on starch, but on albumens and fats. On starch it acts like the saliva, but more energetically. It produces changes in albumens similar to those effected in the stomach, but by the agency of a 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 absorption, † Glycerine also is soluble in water and capable of absorption. The greater part of the fats is not, however, 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 suspended in milk, or olive oil in mayonnaise sauce. 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 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 neighbors 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 color) because the innumerable tiny oil-drops floating in it reflect all the light which falls on its surface.

How is an outpouring of bile on the chyme brought about ? Do liver and pancreas cease secreting when the chyme enters the intestine? What other glands are set to work? How is the acidity of the chyme overcome ? What is chyle ? What does it usually contain?

Describe the pancreatic secretion. What foodstuffs does it act upon? Describe its action on starch. How does it change albumens.

How does trypsin differ from pepsin ? How does pancreatic secretion 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 ? Explain the process. What is an emulsion ? What emulsifies the oily matters of the chyle ?

† 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 fatty acid with potash. Both dissolve in water, which the fats from, which, they are made will not do.