386 A MANUAL OF PHYSIOLOGY 



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 recognise that we do not at present know with any great 

 exactness the mechanism by which the rate of secretion and 

 expulsion 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 mercury.* This is a point of practical 

 importance, for a comparatively slight obstruction to the out- 

 flow, even such as is offered by a congested or inflamed condition 

 of the duodenal wall about the mouth of the duct, may be suffi- 

 cient 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 experi- 

 ment, and that an inconclusive 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-constrictor nerves, for 

 it has the same composition and digestive action as normal 

 succus entericus obtained from a fistula. The secretion begins 

 about four hours after section of the nerves, goes on increasing 



* 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). 



