CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 437 



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 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-suppty 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. 



And as a matter of fact we do find vaso-constrictor action 

 dominant over the secretion. In the various experiments which 

 have been made to ascertain the action .of the nervous system on 

 the secretion of bile, it has always been found that stimulation of 

 the medulla oblongata, or of the spinal cord, or of the abdominal 

 splanchnic nerves, stops or at least checks the flow of bile. Now 

 the effect of these stimulations is, as we have already seen more 

 than once, a powerful constricting action on the abdominal blood 

 vessels ; by such stimulation the blood-supply of the liver is 

 materially diminished, and in consequence the secretory activity 

 is slackened or arrested. 



But there is something besides the mere quantity of blood to 

 be considered in this relation. The blood which passes from the 

 alimentary canal at rest is ordinary . venous blood, laden simply 

 with carbonic acid and the ordinary products of the metabolism 

 of the muscular and mucous coats of the canal. When digestion 

 is going on the portal blood is laden, as we shall see, with some 

 at all events of the products of digestion, with sugar probably 

 and with various proteid bodies. And it is quite possible or even 

 probable that some of these bodies in the portal blood reaching 

 the hepatic cells stir them up to secretory activity ; indeed this 

 view may be regarded as supported by the facts that proteid 

 food increases the quantity of bile secreted, whereas fatty food, 

 which as we shall see passes, chiefly if not wholly, not by the 

 portal vein but by the lymphatics and which is probably largely 

 disposed of in some way or other before it can reach the liver, 

 has no such effect. 



Hence we may infer that at all events the second increase of 



