SECRETION OF PANCREATIC JUICE AND OF BILE. 291 



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. 



224. 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 vasomotor nerves of that canal. As we have already said, in speaking 

 of the vascular system, 155, 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 splanch- 

 nic 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 bloodvessels are con- 

 stricted. 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 constric- 

 tion ; at least we have no evidence supporting any other explanation. 

 Apparently the presence of food in the stomach starts in the mucous mem- 

 brane influences which, ascending to the central nervous system, inhibit the 

 vasomotor 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 defi- 

 nitely settled ; but possibly by the vagus nerve, if it be true, as stated, that 

 centripetal stimulation of that nerve, while it raises the general blood-pres- 

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

 leads to a dilatation of the gastric vessels. So also it is probable that as the 

 food reaches succeeding sections of the alimentary canal, those 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 

 nerves, govern the bloodvessels of consecutive sections of the alimentary canal. 



All this flushing of the canal with blood leads, we repeat, to an increased 

 flow of blood at a higher pressure through the portal vein. Whether there 

 be any additional mechanism set to work, such as, for instance, which some 

 observations suggest, a rhythmical peristaltic contraction of the portal 

 vein, by which the blood is still more rapidly hurried to the liver, and 

 whether the increased venous supply through the portal vein is accompanied 

 by a corresponding increase of the lesser supply of arterial blood through 

 the hepatic artery, is not known. It may, perhaps, be here remarked that 

 there is no need for any increase of arterial blood, since the blood from the 

 alimentary canal, owing to its more rapid passage through the minute vessels, 

 is probably like the corresponding blood in the veins of an active salivary 

 gland (though probably also not to the same extent) less venous than usual 

 during digestion, in spite of the extra quantity of carbonic acid thrown into 

 it by the increased metabolism of the muscular coat during the peristaltic 

 movements. 



