i 4 6 PHYSIOLOGY KOR HliCJINNKkS IIAI-. 



digestion the fats arc divided into exceedingly fine particles, 

 that is, they are emulsified. In milk, fat exists in fine 

 globules, and in this respect milk may be looked upon as an 

 emulsion of fat. If a little liquid fat, olive oil for instance, is 

 briskly shaken up with water, it becomes divided up into 

 globules, but these run together again on standing a few 

 moments, and the oil collects on the surface of the water. If 

 the fat is shaken up with bile instead of with water, the 

 globules do not run together so quickly on standing. If thc^ 

 fat is shaken up with pancreatic juice the fat will remain 

 broken up into fine globules for a longer time still. These 

 experiments show that both bile and pancreatic juice have 

 the power of emulsifying fats, and that this power is the 

 greater in pancreatic juice. The emulsion of fats is then 

 produced chiefly by the direct action of the pancreatic juice. 

 This is, however, assisted by a peculiar chcitiical change which 

 the pancreatic juice by the fat-decomposing ferment spoken of 

 above produces in a small quantity of the fat. The fatty acid 

 set free by this ferment unites with the alkaline sodium salts 

 of the pancreatic juice and bile, and thus forms a soap, as it is 

 called ; ordinary soap being formed by the union of fatty acids 

 with alkalis. So that there comes to be a small amount of a 

 peculiar soap formed in the duodenum. The importance of 

 this is, that when a little, even a very small quantity of a soap 

 is present, the power of the pancreatic juice to emulsify fats is 

 greatly increased, so that under these circumstances, when the 

 juice is mixed with the fat, the latter is divided into ex- 

 ceedingly fine globules. 



Structure of the Small Intestine. The wall of the 

 small intestine consists like that of the rest of the alimentary 

 canal of an internal coat of mucous membrane and an external 

 coat of plain muscular tissue. The muscular fibres of the 

 muscular coat are arranged in two layers. In the internal 

 layer, that next the mucous coat and to which it is adherent 

 by means of connective tissue, the muscular fibres are 

 arranged circularly round the intestine forming the circular 

 layer of muscle, while in the external layer the fibres are 

 disposed along the length of the intestine forming the 

 longitudinal layer of muscle. The external muscular layer is 

 covered by a thin, smooth, transparent membrane, sometimes 



