SECRETION OF PANCREATIC JUICE AND OF BILE. 289 



the secretion of pancreatic juice in having a double rise ; and as in that 

 case so in this, it is very probable that the first rise is in part the result of 

 nervous action, and it is also possible that nervous influences intervene in 

 the second, more lasting rise ; but, as we shall see presently, even nervous 

 influences may affect the liver in a very indirect manner, and our knowl- 

 edge as to any direct action of the nervous system on the liver is at present 

 very imperfect. 



The liver receives its chief nervous supply from the solar plexus, and to a 

 great extent through that part of the solar plexus called the hepatic plexus, 

 which embraces the portal vein, hepatic artery, and bile-duct, as these plunge 

 into the liver at the porta. The solar plexus is fed by the two abdominal 

 splanchnic nerves, major and minor, by other smaller nerves from the lower 

 parts of the splanchnic (sympathetic) chain, and by the terminal portion 

 of the right vagus nerve. Small branches from the left vagus, rami hepatici, 

 also pass directly to the liver from the termination of that nerve on the 

 stomach, finding their way also through the porta. The fibres thus entering 

 the liver from the several sources are, for the most part, non-rnedullated 

 fibres ; with these, however, are mixed a certain number of medullated 

 fibres. 



As to the functions of these nerves in reference to the secretion of bile, 

 we may say at once that no satisfactory or exact statement can at present be 

 made. 



223. It must be remembered, however, that the liver is so peculiarly 

 related to the other organs of digestion, and its vascular arrangements so 

 special that, with regard to it, as compared with many other organs, an in- 

 trinsic nervous mechanism must occupy a more or less subordinate position. 

 The blood-supply of the pancreas for instance is dependent chiefly on the 

 width for the time being of the pancreatic arteries ; it will be affected, of 

 course, by the general arterial pressure and by any circumstances which 

 affect the outflow by the pancreatic veins, and therefore by the condition of 

 the portal venous system of which those veins form a part ; but in the main, 

 the amount of blood bathing the alveoli of the pancreas will depend on 

 whether the pancreatic arteries are constricted or dilated. The quality of 

 the blood reaching the pancreas, being arterial blood drawn direct from the 

 arterial foundation, will be modified only by such circumstances as modify 

 the general mass of the blood. 



Very different is the case of the liver. The supply of arterial blood 

 coming direct through the hepatic artery is small compared with the mass 

 pouring through the vena portse ; it moreover, as we shall see, is distributed 

 in capillaries among the small interlobular branches of the vena portee and 

 has become venous, indeed merged with the portal blood, before it reaches 

 the actual lobules. The supply of blood for the liver is mainly that 

 through the vena porta? ; and this supply is not, like an arterial supply, a 

 fairly uniform one, modified chiefly by the vasomotor events of the organ 

 itself, but is dependent on what happens to be taking place in the alimen- 

 tary canal and in abdominal organs other than the liver itself. When no 

 food is being digested and the alimentary canal is at rest, the vessels of that 

 canal, as we have already said in speaking of the stomach, are like those 

 of the pancreas and salivary glands, in a state of tonic constriction ; a 

 relatively small quantity of blood passes through them ; hence the flow 

 through the vena portee is relatively inconsiderable, and the pressure in 

 that vessel is low. When digestion is going on all the minute arteries of 

 the stomach, intestine, spleen and pancreas are dilated, and general arterial 

 pressure being by some means or other maintained (see 180), a relatively 

 large quantity of blood rushes into the vena portse and the pressure in that 



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