292 THE TISSUES AND MECHANISMS OF DIGESTION. 



225. It is interesting to observe that the pressure under which the bile 

 is secreted is relatively low, like that of the pancreatic juice, not high like 

 that of the saliva ; it is much lower than the arterial pressure in the same 

 animal, whereas in the case of saliva ( 197) the pressure is greater than the 

 blood-pressure in the carotid artery. But, in the case of bile, since the blood 

 which flows through the hepatic lobules is, mainly, venous portal blood, we 

 have to compare the pressure of the secretion, not with arterial pressure but 

 with the venous pressure in the portal system ; and in the dog it has been 

 found that while the pressure of the bile secreted stood at about 200 mm. of 

 a solution of sodium carbonate that is, about 15 mm. mercury the blood- 

 pressure in a branch of the superior mesenteric vein stood only at about 90 

 mm. of the same solution that is, about 7 mm. mercury. Now, the venous 

 pressure in the mesenteric veins is higher, though only slightly higher, than 

 that in the portal vein into which these pour their blood (the difference of 

 pressure being the main cause why the blood flows from the one into the 

 other), and is, therefore, certainly higher than the pressure in the portal 

 capillaries of the hepatic lobules. So that what is true of the salivary gland 

 is also true, on a different scale, of the liver, viz., that the pressure exerted 

 by the secretion is higher than the pressure of the blood in the vessels feed- 

 ing the secreting cells. 



226. If the pressure in the bile-duct be artificially increased, as by 

 pouring fluid into the glass tube or manometer with which the canula in the 

 duct is connected, a resorption of the secreted bile takes place ; and resorp- 

 tion will also take place within the body, when the pressure generated by the 

 act of secretion itself reaches and is maintained at a sufficiently high level. 

 Thus, when in the living body the bile-duct is ligatured, or becomes ob- 

 structed by gall-stones or otherwise, fluid is accumulated on the near side of 

 the ligature at a pressure which goes on increasing until resorption of the 

 bile takes place, bile salts and biliary pigments are thrown back upon the 

 system, and "jaundice" results. It would appear that in these cases re- 

 sorption takes place through the interlobular bile-ducts and not through the 

 hepatic cells or other structures within the lobules. The high pressure in 

 the ducts does not lead to a reversal of the current in the hepatic cells (at 

 most it slackens or possibly stops the current), but the bile secreted into the 

 interlobular ducts escapes from these. It further appears that the escape is 

 not into the bloodvessels but into the lymphatics ; the bile salts, pigments, 

 and other constituents are carried into the thoracic duct, and in an indirect 

 manner only find their way into the blood stream. 



To complete the history of the secretion of bile we ought now to turn to 

 the manufacture of the biliary constituents within the cells. But since the 

 hepatic cells are also engaged in labors other and more important, perhaps, 

 than that of secreting bile, it will be convenient to defer what we have to 

 say on this point until we come to speak of the formation of glycogen and 

 of the general metabolic events taking place in the liver. 



THE MUSCULAR MECHANISMS OF DIGESTION. 



227. From its entrance into the mouth until such remnant of it as is 

 undigested leaves the body, the food is continually subjected to movements 

 having for their object the tritu ration of the food as in mastication, or its 

 more complete mixture with the digestive juices, or its forward progress 

 through the alimentary canal. These various movements may briefly be 

 considered in detail : 



228. Peristaltic movements. Putting aside the somewhat complicated 

 pharyngeal part of deglutition, and taking the resophageal movements by 



