Chap, i.] TISSUES AND MECHANISMS OF DIGESTION. 369 



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

 nant 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 ma- 

 terially diminished, and in consequence the secretory activity is 

 slackened or arrested. 



But there is something b3sides 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 

 the flow of bile which occurs during the later stages of digestion 

 may be to a large extent the direct effect of blood, laden with 

 digestive products, passing from the stomach and intestines, espe- 

 cially the latter, to the liver by the portal vein, quite independent 

 of any direct nervous action on the liver itself ; and indeed it is 

 possible that the first rise also may be partly due to the increased 

 flow of blood from the stomach, aided by the absorption from that 

 organ of a certain amount of digested material. Since, however, 

 there is no evidence of any decrease in blood-supply, or in the rate 

 of absorption, corresponding to the fall between the two rises, 

 some influences other than those which we are discussing must 

 be at work in the matter. 



§ 216. It is interesting to observe that the pressure under 

 which the bile is secreted is relatively low, not high like that of 

 the saliva ; it is much lower than the arterial pressure in the same 

 animal, whereas in the case of saliva (§ 189) the pressure is greater 

 than the blood-pressure in the carotid artery. But, in the case 

 of bile, since the blood which flows through the hepatic lobules 

 is, mainly, venous portal blood, we have to compare the pressure 

 of the secretion not with arterial pressure but with the venous 



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