CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 435 



stomach are poured over the orifice of the biliary duct, a gush of 

 bile takes place. Indeed, stimulation of this region of the duo- 

 denum with a dilute acid at once calls forth a flow, though 

 alkaline fluids so applied have little or no effect. When no such 

 acid fluid is passing into the duodenum no bile is, under normal 

 circumstances, discharged into the intestine. The discharge is due 

 to a contraction of the muscular walls of the gall-bladder and 

 ducts, accompanied by a relaxation of the sphincter of the orifice ; 

 both acts are probably of a reflex nature, but the details of the 

 mechanism have not been worked out. 



The secretion of bile on the other hand, as shewn by the 

 results of biliary fistula, is continuous ; it appears never to cease. 

 When no food is taken the bile passes from the liver along the 

 hepatic and then back along the cystic duct (the flow being aided 

 probably by peristaltic contractions of the muscular fibres of the 

 duct) to the gall-bladder, where it is temporarily stored ; hence in 

 starving animals, when no discharge is excited by food, the gall- 

 bladder becomes greatly distended with bile. But the secretion, 

 though continuous, is not uniform. The rate of secretion varies, 

 and is especially influenced by food ; it is seen to rise rapidly after 

 meals, reaching its maximum, in dogs, in from four to eight hours. 

 There seems to be an immediate, sudden rise when food is taken, 

 then a fall, followed subsequently by a more gradual rise up to 

 the maximum, and ending in a final fall to the lowest point. 

 The curve of secretion, in fact, resembles that of 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 knowledge 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 

 terminations 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-medullated 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. 



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