438 BLOOD SUPPLY OF LIVER. [BOOK n. 



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, 

 especially the latter, to the liver by the portal vein, quite 

 independent of any direct nervous action on the liver itself; 

 and indeed itf 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. 



254. The blood-supply of the liver being thus, quite apart 

 from any nervous supply of its own, so closely dependent on what 

 is going on in the alimentary canal, it will be convenient to say a 

 few words more concerning the vaso-motor nerves of that canal. 

 As we have already said in speaking of the vascular system 

 ( 169), the vaso-constrictor fibres for the stomach and intestines, 

 large and small, issuing from what we may call the vaso-constrictor 

 region of the cord pass for the most part through the two 

 abdominal splanchnic nerves, major and minor, a small number 

 only passing out below the roots of those nerves. When these 

 splanchnic nerves are divided the vessels of the canal are dilated, 

 when they are centrifugally stimulated the vessels are constricted. 

 Whether there be any distinct vaso-dilator fibres for all or any part 

 of the canal, and if so what course they take, is not known. When 

 no food has for some time been taken, the mucous membrane of the 

 stomach as seen through a gastric fistula is pale ; the blood vessels 

 are constricted. And as far as we know a similar condition obtains 

 throughout the small and large intestines. When food is taken 

 the mucous membrane of the stomach becomes flushed ; its vessels 

 become dilated. This appears to be the result of an inhibition of 

 the previously existing tonic constriction; at least we have no 

 evidence supporting any other explanation. Apparently the 

 presence of food in the stomach starts in the mucous membrane 

 influences which, ascending to the central nervous system, inhibit 

 the vaso-motor centre for the abdominal splanchnic nerves or such 

 part of that centre as governs the vaso-constrictor fibres of the 

 stomach. By what path such afferent impulses reach the central 

 nervous system is not as yet definitely settled; but possibly by 

 the vagus nerve, if it be true, as stated, that centripetal stimu- 

 lation of that nerve, while it raises the general blood-pressure 

 by increasing, in a reflex manner, vaso-constriction in other 

 regions, leads to a dilation of the gastric vessels. So also it 

 is probable that as the food reaches succeeding sections of 

 the alimentary canal, these in turn in a similar manner become 

 flushed with blood. In the frog there is some evidence that 

 vaso-constrictors leaving the spinal cord by consecutive spinal 



