CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 461 



moreover, as we shall see, is distributed in capillaries among the 

 small interlobular 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 portae. Now, as we have said ( 180), 

 there is evidence that the condition of the muscular walls of this 

 great vein, and hence the calibre of the vein, is governed by 

 impulses passing along the splanchnic nerves, so that the supply 

 of blood to the liver by the portal vein may be directly governed 

 by the central nervous system, much in the same way as is the 

 ordinary arterial supply to this or that organ. Making every 

 allowance however for this special influence, by which we may 

 remark not only the flow of blood to the liver but the pressure 

 in the mesenteric and other veins and capillaries is affected, and 

 which indeed seems to have for its purpose the accommodation 

 of the portal vein to the flow of blood through the viscera, we 

 may still conclude that the flow of blood along the portal vein 

 and so the supply of blood to the liver is in the main 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 portae is rela- 

 tively small, 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 

 the central nervous system by a single nervous tie, the tide 

 of blood through the liver would ebb and flow according to the 

 absence or presence of food in the alimentary canal. 



An increase of blood-supply does not of course necessarily 

 mean an increase of secretory activity. As we have seen, 227, in 

 the presence of atropin the secretion of saliva may stand still in 

 spite of dilated bloocj vessels and the consequent rush of blood ; 



