=51.= The bile and the pancreatic juice together act upon all fats in a very curious way. You know that if you shake up oil and water together, though by violent shaking you may mix them a good deal, directly you leave off they separate again, and all the oil is seen floating on the top of the water. If, however, you shake up oil with pancreatic juice and bile, the oil does not separate. You get a sort of creamy mixture, and will have to wait a very long time before the oil floats to the top. Milk, you know, contains fat, the fat which is generally called butter. If you examine milk under the microscope, you will find that the fat is all separated into the tiniest possible drops. So also, when you shake up oil or butter, or any other fat, with bile and pancreatic juice, you will find on examination that the fat or oil is all separated into the tiniest possible drops. What is the purpose of this? If you look at the inside of the small intestine of any animal, you will find that it is not smooth and shiny like the outside of the intestine, but shaggy, or, rather, velvety. This is because the mucous membrane is crowded all over with little tags, like very little tongues, hanging down into the inside of [Illustration: FIG. 18.--_Semi-diagrammatic View of Two Villi of the Small Intestines._ (Magnified about 50 diameters.) _a_, substance of the villus; _b_, its epithelium, of which some cells are seen detached at _b_^{2}; _c d_, the artery and vein, with their connecting capillary network, which envelopes and hides _e_, the lacteal which occupies the centre of the villus and opens into a network of lacteal vessels at its base. ]