436 BLOOD-SUPPLY OF LIVER. [BOOK n. 



253. 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 ^he 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 inter- 

 lobular branches of the vena portse 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 portse ; and this supply is not, like an arterial supply, a 

 fairly uniform one, modified chiefly by the vaso-motor events of 

 the organ itself, but is dependent on what happens to be taking 

 place in the alimentary 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 portse 

 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 194), 

 a relatively large quantity of blood rushes into the vena portse 

 and the pressure in that vessel becomes much increased, though 

 of course remaining lower than the general arterial pressure. 

 Moreover during digestion, peristaltic movements of the muscular 

 coats of the alimentary canal are, as we have seen, active; and 

 these movements, serving as aids to the circulation (see 121), 

 help to increase the portal flow. Further the spleen, as we 

 shall see in speaking of that organ, is in many animals richly 

 provided with plain muscular fibres, and in such cases seems, 

 especially during digestion, to act as a muscular pump driving 

 the blood onwards, with increased vigour, along the splenic veins 

 to the liver. So that even were the liver not connected with 



