CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 439 



nerves govern the blood vessels 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 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 

 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. 



255. It is interesting to observe that the pressure under 

 which the bile is secreted is relatively low like that of the 

 pancreatic juice, 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 ( 227) 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 pressure in the portal system ; and 

 in the dog it has been found that while the pressure of the bile 

 secreted stood at about 200 mm. of a solution of sodium carbonate, 

 that is, about 15 mm. mercury, the blood-pressure in a branch of 

 the superior mesenteric vein stood only at about 90 mm. of the 

 same solution, that is, about 7 mm. mercury. Now the venous 

 pressure in the mesenteric veins is higher, though only slightly 

 higher, than that in the portal vein into which these pour their 

 blood (the difference of pressure being the main cause why the 

 blood flows from the one into the other), and is therefore certainly 

 higher than the pressure in the portal capillaries of the hepatic 

 lobules. So that what is true of the salivary gland is also true, 

 on a different scale, of the liver, viz. that the pressure exerted by 

 the secretion is higher than the pressure of the blood in the vessels 

 feeding the secreting cells. 



256. If the pressure in the bile duct be artificially increased, 

 as by pouring fluid into the glass tube or manometer with which 

 the cannula in the duct is connected, a resorption of the secreted 

 bile takes place ; and resorption will also take place within the 

 body, when the pressure generated by the act of secretion itself 

 reaches and is maintained at a sufficiently high level. Thus 



