338 A MANUAL OF PHYSIOLOGY 



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

 man is probably from 750 c.c. to a litre. In nine cases of fistula 

 of the gall-bladder in patients operated on for gall-stones or 

 echinococcus the daily quantity varied from 500 to 1,100 c.c. 

 (Brand). 



The great action of the bile in digestion is undoubtedly the 

 preparation of the fats for absorption. In this preparation four 

 processes are important : two chemical actions, hydrolysis of 

 neutral fats to glycerin and fatty acids, and saponification, or 

 the formation of soaps by the union of fatty acids with bases, 

 especially sodium ; and two physical processes, emulsification, 

 or the formation of a mechanical suspension of such fine globules 

 of unaltered neutral fat as exist in milk, and solution of soaps and 

 fatty acids. While there has been much discussion as to the 

 relative share taken by these processes, and especially by 

 saponification and emulsification in the absorption of fat (p. 412), 

 there is no doubt that they are all concerned in the digestion of 

 fat or the preparation of it for absorption. In this, indeed, the 

 processes are complementary to each other, for an essential pre- 

 liminary to emulsification in the intestine seems to be the for- 

 mation of a certain amount of soaps, soluble in the intestinal 

 contents, 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 and solution. In 

 the whole series of changes the bile plays a part, though not an 

 independent one ; it acts always in conjunction with the pan- 

 creatic juice. 



While no complete explanation has been given of the precise 

 nature of this partnership, 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, emulsification is rapid and complete (p. 12). 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 

 forming 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 ferment which can split 

 up neutral fats and set free fatty acids, and an alkali which can 

 combine with those acids to form soaps. Accordingly, pancreatic 



