340 STUDIES IN ADVANCED PHYSIOLOGY. 



the fatty acids so produced in the intestine, unite with some 

 of the alkaline ingredients present and form soap. It can 

 be easily shown in a chemical laboratory that the formation 

 of soap is practically impossible when a perfectly pure fat 

 is used. Traces of a fatty acid must be present to help the 

 process along. In the intestine this fatty acid is produced 

 by the steapsin and so the formation of soap rendered pos- 

 sible and simple. 



The question then at once arises, what the physiological 

 value of soap is in the intestine. In the first place, soap is 

 soluble and dialyzable, while ordinary fat is not very dialyz- 

 able, and so if the fats are changed into soap it will make 

 quite easy the transfer of these substances into the blood. 

 The same is true of glycerine also, which is always formed 

 when the fats split up into a fatty acid. 



The main purpose, however, of the soap is probably to 

 aid in the emulsification of the remaining fats. It was 

 pointed out in discussing the action of the gastric juice on 

 fats that all the fats were liberated. In the intestine they 

 are prepared for absorption. A small per cent, of them, as 

 just indicated, is changed into fatty acid, and then into 

 soap. The rest is emulsified. An emulsification of an oil 

 or fat is effected when the oil in question is shaken up very 

 thoroughly with some liquid, so that the two seem to have 

 been intimately mixed and the fat is prevented from run- 

 ning together by being suspended in tiny droplets sur- 

 rounded by a thin envelope to prevent them reuniting. 

 Thus, cream is an emulsion of butter, the tiny particles of 

 butter being surrounded by an albuminous covering and so 

 prevented from running together. Lather is an emulsion 

 of air and soap-suds. The significance of the emulsifica- 

 tion of the fats lies in the fact that the bits of fat in the in- 

 testine are separated into very tiny particles small enough 

 to pass through the walls of the alimentary canal in a 

 manner to be described in the following chapter. In large 

 chunks or bits this would be impossible, but reduced to 



