806 PHYSIOLOGY 



rate of secretion was therefore doubled. We may conclude that 

 the mechanism, by which the increased secretion of bile is pro- 

 duced at the time when this fluid is required in the intestine, is 

 identical with that for the secretion of pancreatic juice, and that 

 in each case one and the same substance- secretin is formed by 

 the action of the acid on the cells of the mucous membrane, and, 

 on absorption into the blood stream, excites both the liver and 

 pancreas to increased activity. 



THE DIGESTIVE FUNCTIONS OF THE BILE 



Bile contains a weak amy lory tic ferment. Its uses in digestion 

 are dependent, however, not on the presence of this ferment, but 

 on the peculiar action of the bile salts on the fermentative powers 

 of the pancreatic juice. It was shown long ago by Williams and 

 Martin that the aniylolytic power of pancreatic extracts is doubled 

 by the addition of bile or of bile salts. Pawlow has stated that the 

 same holds good of the proteolytic power of this juice. Most 

 important, however, is the part played by the bile in the digestion 

 and absorption of fats. The fat- splitting action of pancreatic juice 

 is trebled by the addition of bile, whether boiled or unboiled. This 

 quickening action of the bile probably depends, like its function in 

 the absorption of fats, on the peculiar physical properties of the 

 bile salts, with those of the lecithin and cholesterin which they hold 

 in solution. Not only does such a solution diminish the surface 

 tension between watery and oily fluids, so promoting the closer 

 approach by the lipase of the pancreatic juice to the fats on which 

 it is to act, but it has also the power of dissolving fatty acids and 

 soaps, including even the insoluble calcium and magnesium soaps. 

 It is probable that it aids also in holding in solution, and bringing 

 in contact with the fat, the lipase of the pancreatic juice. It has 

 been shown by Nicloux that the lipase contained in oily seeds, such 

 as those of the castor plant, is insoluble in water, but soluble in fatty 

 media. The dried ferment obtained from the pancreas in many 

 cases yields no lipase to water, but gives a strongly lipolytic solution 

 when extracted with glycerin. The digestive function of bile there- 

 fore lies in its power of serving as a vehicle for the suspension and 

 solution of the interacting fats, fatty acids, and fat-splitting ferment. 

 This vehicular function plays an important part in the absorption 

 of fats. These pass through the striated basilar membrane bounding 

 the intestinal side of the epithelium, not, as has been formerly thought, 

 in a fine state of suspension (an emulsion), but dissolved in the bile 

 in the form of fatty acids or soaps and glycerin. On the arrival of these 

 products of digestion in the epithelial cells, a process of resynthesis 

 is set up. Droplets of neutral fat make their appearance in the cells, 



