When fat is cooked in the food, a good deal of it is converted into rancid fat - i. e., fat containing free fatty acids. In the mouth the fatty food is unaltered, unless it be in the form of adipose tissue, when the process of mastication breaks it down and liberates some of the fat globules, which are melted by the natural warmth. If eaten in the form of oil or butter, with starchy or vegetable food, the fat coats the particles of food in the mouth. Bread thus mingled with butter is less likely to form a large and too tenacious bolus.

In the stomach the fat remains unaltered, and when the semifluid chyme begins to flow into the intestine it passes in with it. The connective-tissue elements of fatty meats, etc., are dissolved away and digested like other proteids by the gastric juice.

In the intestine the fat is subjected to a double process of emul-sification and saponification, which is accomplished by the combined action of the bile and pancreatic juice. The latter splits fats into glycerin and fatty acid, and this action is favoured by the admixture with bile, which simultaneously occurs in the duodenum. According to Rachford, the fat-splitting power of pancreatic juice is more than quadrupled by bile, although the latter has no fat-splitting influence of its own. He also declares that the presence of hydrochloric acid which has just entered the upper duodenum with the chyle, like the bile, still further aids the pancreatic action upon fats, as four to one.

It is held by the majority of authorities that pancreatic juice also contains an emulsifying ferment, steapsin, and that the free fatty acid above described splits up the sodium bile salts and liberates their sodium, which unites with a portion of the free fatty acid, forming a soap. The soap coats the droplets of emulsified fat with a thin film (like a soap bubble) and prevents them from coalescing again. The emulsion is then absorbed by the lacteals.

It is, however, not a necessary part of the theory of digestion of fatty food that the fat must all be emulsified. No doubt a large part of it is absorbed more directly or as a soap without emulsification.

Extracts of both the liver and spleen have been said to favour the absorption of fats by emulsification.

Undigested food reaches the intestines under the following conditions: (a) When too much food is eaten, (b) when gastric digestion is imperfect, (c) when the food is improper.