Chap, i.] TISSUES AND MECHANISMS OF DIGESTION. 367 



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 

 oritice ; 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 flstuhe, 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. 



§ 215. 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 intrinsic 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 porta? ; it moreover, as 

 we shall see, is distributed in capillaries among the small inter- 



