368 BLOOD-SUPPLY OF LIVER. L Boox u. 



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 cf 

 blood passes through them ; hence the flow through the vena port* 

 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 172), 

 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 pressuie. 

 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 103). 

 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, 189, 

 in the presence of atropine, the secretion of saliva may stand still 

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

 but we may safely assert that, other things being equal, a fuller 

 blood-supply is favourable to activity. Apparently a mere change 

 in the quantity of blood bathing an alveolus will not start in the 

 cells the changes which constitute the act of secretion, any more 

 than an increase in the blood bathing a muscular fibre will neces- 

 sarily set going a contraction ; but unless there be some counter- 

 acting influence at work, a fuller and richer lymph around a cell 

 will naturally lead to the cell taking up more material from the 

 lymph, and so will increase the cell's store of energy. Hence ; 

 especially in the hepatic cell, which appears to be always at 

 work, always undergoing metabolism of such a kind as to give 

 rise to bile, we might fairly expect the greater flow through the 

 portal vein to quicken the flow through the bile duct. 



