4*4 DIGESTION 



flow of bile-salts into the intestine accelerates the splitting of fats by 

 the pancreatic juice, and therefore the absorption of bile- salts acting 

 as solvents for, or chemically united to, the fatty acids and soaps. 

 A circle analogous to the ' vicious circle ' of the logicians, but con- 

 stituting a physiological adaptation of most potent virtue in the 

 digestion of fats, is thus established. Not only is the quantity of 

 bile poured into the intestine increased on a diet rich in fat, but it 

 is said that a given amount of it aids the fat-splitting action of the 

 pancreatic juice more powerfully than if the diet were poor in fat. 

 This may depend upon an increase in the concentration of the bile- 

 salts in bile secreted when a large amount of fat is ingested. But it 

 is well to recognize that we do not at present know with any great 

 exactness the mechanism by which the rate of secretion and ex- 

 pulsion of bile and the properties of that juice are influenced by 

 digestion. It has been conjectured that the first abrupt rise may be 

 started by reflex nervous action, and that later on secretin and, in the 

 case of fat digestion, bile-salts may directly excite the hepatic cells. 



The pressure under which the bile is secreted is higher than the 

 pressure of the portal blood, and therefore the liver ranges itself with 

 the high-pressure salivary glands rather than with the low-pressure 

 pancreas. But although the biliary pressure is high relatively to 

 that of the blood with which the secreting cells are supplied, it is 

 absolutely low, the maximum being no more than 25 mm. of mer- 

 cury.* This is a point of practical importance, for a comparatively 

 slight obstruction to the outflow, even such as is offered by a con- 

 gested or inflamed condition of the duodenal wall about the mouth 

 of the duct, may be sufficient to cause reabsorption of the bile 

 through the lymphatics, and consequent jaundice. Of course, 

 complete plugging of the duct by a biliary calculus is a much more 

 formidable barrier, and inevitably leads to jaundice, just as ligature 

 of a salivary duct, in spite of the great secretory pressure, inevitably 

 causes oedema of the gland. 



The Influence of Nerves on the Secretion of Intestinal Juice. As 

 to the influence of nerves on the secretion of the succus entericus, our 

 knowledge is almost limited to a single experiment, and that an in- 

 conclusive one. Moreau placed four ligatures on a portion of the 

 small intestine, so as to form three compartments separated from 

 each other and from the rest of the gut. The mesenteric nerves 

 going to the middle loop were divided, and the intestine returned to 

 the abdomen. After some time a watery secretion was found in the 

 middle compartment, little or none in the others. This is a true 

 ' paralytic ' secretion, and not a mere transudation depending 

 simply on the vascular dilatation caused by section of the vaso- 



* In the dog, cat, and monkey the average maximum pressure at which 

 as much bile is secreted as is taken up from the bile-paths by the portal 

 lymphatics is about 300 mm. of bile. The highest pressure recorded was 

 373 mm. of bile in a cat (Herring and Simpson). 



