CHEMISTRY OF DIGESTION AND NUTRITION. 245 



among them butyric acid, which is readily recognized by its familiar odor, 

 that of rancid butter. The action of steapsin, as in the case of the other 

 enzymes, is very much influenced by the temperature. At the body-temper- 

 ature the action is very rapid. The nature of the fat also influences the 

 rapidity of the reaction; it may be said, in general, that fats with a high 

 melting-point are less readily decomposed than those with a low melting- 

 point. It has been shown, however, that even spermaceti, which is a body 

 related to the fats and whose melting-point is 53 C., is decomposed, although 

 slowly and imperfectly, by steapsin. The fat-splitting action of the steapsin 

 undoubtedly takes place normally in the intestines, but it must not be supposed 

 that all the fat eaten undergoes this process. On the contrary, it is believed 

 that a small portion only of the fats and oils is affected by the steapsin, by far 

 the larger portion remaining unaffected and being absorbed into the blood as 

 neutral fat. What, then, is the physiological value of steapsin in the digestion 

 and absorption of fats? This question is difficult to answer satisfactorily if 

 one goes into the details. In general, however, it is commonly taught that the 

 small part of the fat split by the steapsin into fatty acid and glycerin helps 

 to emulsify the balance of the fat and thereby renders its absorption possible. 

 The fat-splitting action of steapsin, then, is of indirect value in digestion, and 

 its importance can be brought out best by describing the em unification of fats 

 and the conditions bringing this emulsification about. 



Emulsification of Fats. An oil is emulsified when it is broken up into 

 minute globules which do not coalesce, but which remain separate and more 

 or less uniformly distributed throughout the medium in which they exist. 

 Artificial emulsions can be made by shaking oil vigorously in viscous solutions 

 of soap, mucilage, etc. Milk is a natural emulsion which separates partially 

 on standing, some of the oil rising to the top to form cream. Bernard made 

 the important discovery that when oil and pancreatic juice are shaken together 

 an emulsion of the oil takes place very rapidly, especially if the temperature 

 is about that of the body. The main cause of the emulsification has been 

 shown to be the formation of free fatty acids due to the action of steapsin, 

 and the union of these acids with the alkaline salts present to form soaps. 

 This fact has been demonstrated by experiments of the following character : 

 If a perfectly neutral oil is shaken with an alkaline solution (J per cent, 

 sodium-carbonate solution), no emulsion occurs and the two liquids soon sepa- 

 rate. If to the same neutral oil one adds a little free fatty acid, or if one 

 uses rancid oil to begin with and shakes it with J per cent, sodium-carbonate 

 solution, an emulsion forms rapidly and remains for a long time. Oil con- 

 taining fatty acids when shaken with distilled water alone will not give an 

 emulsion. It has been shown, moreover, by Gad. and Eatchford that with a 

 certain percentage of free fatty acids (5J per cent.) rancid oil and a sodium- 

 carbonate solution will form a fine emulsion spontaneously that is, without 

 shaking. Shaking, however, facilitates the emulsification when the amount 

 of free acid varies from this optimum percentage. In what way the formation 

 of soaps in an oily liquid causes the oil to become emulsified is still a matter 



