THE INNER TISSUES OF ANIMALS. 329 



and its vessels when it has them. But these vessels and 

 tissues are elastic; and if distended must everywhere com- 

 press their contents — must tend, therefore, to squeeze out their 

 contents where there is least resistance. Consequently, if at 

 any place there is an abstraction of nutritive liquid, either 

 for growth or function, more nutritive liquid will be forced 

 towards that place. This cause of currents, which cannot 

 fail to work throughout the distended tissues even of animals 

 that are without blood-vessels, comes more actively into play 

 where the body is everywhere traversed by these branching 

 tubes with elastic walls. When we learn that the pressure 

 of blood within the arteries and veins of a mammal varies 

 from some 3 lbs. to \ of a lb. per square inch, we see, on 

 averaging this pressure, that the coats of the vascular system 

 exert considerable force on the blood. This average pressure 

 cannot be due to the heart's action ; since if, in the absence 

 of the heart's action, the whole mass of the blood in 

 the vascular system were not above atmospheric pressure, 

 the heart's action could not produce a pressure above 

 that of the atmosphere in one part of the vascular system 

 without lowering the pressure below that of the atmo- 

 sphere in another part of the vascular system. Hence 

 it follows that irrespective of the heart's action, the dis- 

 tended walls of the vascular system must so compress 

 the blood, as to cause a flow of it towards places 

 where its escape is least resisted — towards places, that is, 

 where it is most rapidly abstracted by function or growth. 

 This is a cause of distribution which is at work before any 

 central organ of circulation exists. Though in the rudimen- 

 tary vascular systems of the simpler animals, the osmotic 

 distension is probably nothing like so great, there must 

 be some of it ; and in the absence of a pumping organ, 

 this force is probably an important aid to that move- 

 ment of the blood which the functions set up. How 

 the third cause — the changes of internal pressure which an 

 animal's movements produce — furthers circulation, will be 



