THE CHEMISTRY OF THE DIGESTIVE JUICES 361 



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 echino- 

 coccus the daily quantity varied from 500 to 1,100 c.c. (Brand). 



Digestive Functions of Bile. 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, emulsifi- 

 cation, 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 saponi- 

 fication and emulsification in the absorption of fat (p. 435), there is 

 no doubt that they are all concerned in the digestion of fat or the 

 preparation of it for absorption and assimilation. In this, indeed, 

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

 liminary to emulsification in the intestine seems to be the formation 

 of a certain amount of soaps, soluble in the intestinal contents, while 

 the formation of an emulsion enormously increases the surface of 

 contact between the unaltered fat and the digestive juices, and so 

 favours more rapid hydrolysis, saponification, and solution. In the 

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

 dent one; it acts always in conjunction with the pancreatic 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, emulsifi- 

 cation 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 juice is able of itself to form emulsions with perfectly 

 neutral oils. It is possible that the protein constituents of pancreatic 

 juice may have a share in emulsification, since the addition of protein 



