CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 463 



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 may be well to recall to 

 mind what has been stated (179) concerning the vascular changes 

 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 spinal cord, pass for the most part 

 through the two 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. 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 so 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 ; and a similar flushing with blood 

 appears to occur in the intestines. Now, though indirect evidence 

 has been offered that the splanchnic nerves contain some vaso-dilator 

 fibres, we cannot consider this flushing as a vaso-dilator effect similar 

 to that which is so conspicuous in the sub-maxillary and other 

 salivary glands. Indeed it may, in part at least, be independent 

 of the central nervous system ; the dilated condition of the blood- 

 vessels may be due to some local action of the presence of food, may 

 be the direct result of the activity of the parts, much in the same 

 way as, according to some views, the dilated condition of the blood- 

 vessels of a muscle is due to a direct action of the products of 

 muscular activity ( 168). But, so far as it is due to some 

 intervention of the central nervous system, it is brought about, we 

 may infer, by an inhibition at its centre of the vaso-constrictor 

 mechanism referred to above; afferent impulses started in the 

 mucous membrane pass centripetally to the central nervous system, 

 travelling possibly along afferent vagus nerves, though this has 

 not been definitely made out, and inhibit within the central nervous 

 system that part of the vaso-constrictor mechanism which governs 

 the vascular tone 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 besides this 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 



