CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 531 



arteries. At the moment that the constriction is effected a certain 

 extra quantity of blood is thereby driven on into the capillaries, 

 and raises the pressure in them ; but this is quite temporary and 

 may be neglected. We may say then that the artery feeding a 

 capillary area remaining unchanged in calibre, the pressure in the 

 capillary will depend on the mean arterial pressure ; and that the 

 mean arterial pressure remaining the same, the pressure in the 

 capillaries will rise when the artery feeding them is dilated and will 

 fall when it is constricted. 



But the capillary pressure may be modified by events on the 

 venous side. To take an extreme case; if the vein (or veins) 

 draining a capillary area be quite occluded, the pressure in the 

 vein and in the capillaries rises until it reaches the height of that 

 of the artery or rather of the mouth of the artery feeding the area; 

 under these circumstances there is a level of pressure throughout 

 vein, capillaries and artery, and there is in consequence no flow of 

 blood along that tract, for the blood only flows from a higher to a 

 lower level of pressure. So also when the flow along the vein 

 is not actually arrested but only hindered, an increase of venous 

 pressure, less in amount, is the result, and this, working backwards, 

 increases the capillary pressure. Thus capillary pressure may be 

 raised by whatever tends to increase venous pressure. 



Now speaking at first of the veins of the body generally and 

 putting aside accidental and mechanical causes, such as local pres- 

 sure from a tumour, the plugging of a vein, and the like, we may 

 say that there are, so far as we know, two main causes leading 

 to an increase of venous pressure, namely, obstacles to the passage 

 of blood through the heart from the venous to the arterial side, and 

 a sudden increase in mass of the circulating blood, since the excess 

 is in that case, as we have seen, lodged in the venous system 

 ( 186). In the blood vessels of the abdominal viscera a special 

 factor intervenes. As we have seen the portal vein and its branches 

 are subject to vaso-constrictor impulses passing down the splanchnic 

 nerves. Hence quite apart from arterial changes, or from general 

 venous changes, the pressure in the capillaries of the alimentary 

 canal and of the organs from which the portal vein draws its 

 contents may be raised by a constriction of the portal vein (and 

 its branches). It may be noted that another effect of this same 

 constriction is to diminish the pressure in the capillaries of the 

 liver. 



We may now ask the question, Does increase of the pressure 

 in the capillaries increase transudation ? We have abundant 

 evidence that it does. 



The amount of lymph passing into the lymph-spaces and so the 

 flow of lymph along the lymph-channels is increased when the 

 capillary pressure is raised by the arteries being dilated. Thus if, 

 in a dog, cannulse having been placed in the lymphatic vessels 

 leading from each of the hind feet, the sciatic nerve on one side be 



