DIGESTION 313 



The quantity of bile secreted in twenty-four hours in an 

 average man is probably from 750 c.c. to a litre. 



The great action of the bile in digestion is undoubtedly 

 the preparation of the fats for absorption. In this prepara- 

 tion two processes are important : a chemical process, saponi- 

 fication, or the formation of soaps from the fatty acids of 

 decomposed neutral fats, and a physical process, emulsification, 

 or the formation of a mechanical suspension of such fine 

 globules of unaltered neutral fat as exist in milk. While 

 there has been much discussion .as to the relative share taken 

 by saponification and emulsification in the absorption of fat 

 (p. 370), there is no doubt that they are both concerned in 

 the digestion of fat or the preparation of it for absorption. 

 In this, indeed, the two processes are complementary, for an 

 essential preliminary to emulsification in the intestine seems 

 to be the formation of a certain amount of soaps, while the 

 formation of an emulsion at least increases the surface of 

 contact between the unaltered fat and the digestive juices, 

 and so favours more rapid saponification. In both processes 

 the bile plays a part, though not an independent one ; it 

 acts always in conjunction with the pancreatic juice. 



No completely satisfactory explanation has been given of 

 the precise nature of this partnership, but it is certain that 

 the fat-splitting ferment of the pancreatic juice, on the one 

 hand, and the bile-salts on the other, contribute largely to 

 the total action. An alkaline solution, a solution of sodium 

 carbonate, e.g., is unable of itself to emulsify a perfectly 

 neutral oil ; but if some free fatty acid be added, emulsifica- 

 tion is rapid and complete. Now, there is no doubt that 

 here a soap is formed by the action of the alkali on the fatty 

 acid, and there is equally little doubt that the formation of 

 the soap is an essential part of the emulsification. But it is 

 not clear in what manner the soap acts, whether by form- 

 ing a coating round the oil-globules, or by so altering the 

 surface tension, or other physical properties of the solution 

 in which it is dissolved, that they no longer tend to run 

 together. However this may be, in pancreatic juice we have 

 the two factors present which this simple experiment shows 

 to be necessary and sufficient for emulsification ; we have a 



