MI.M..M: XXVI.J < 'A PI LI.A HY ATTRACTION, I 



towards that in the hepatic veins. No regurgitation can 

 take place backwards through the portal vein upon the 

 blood arising from the chylopoietic viscera, because along 

 that channel there is a pressure propagated in the oppo- 

 site direction arising from the arterial blood of the aortic 



O 



branches. This pressure conspires with that of the por- 

 tal blood, and both together join in giving rise to mo- 

 tion towards the ascending cava. 



On the same principle we may explain the circulation 

 of the blood in other types of life ; for example, in the 

 case of the model adopted in fishes, the aorta of which 

 has long been recognized as bearing a strong resem- 

 blance to the portal vein of the mammalia. To any one, 

 however, who reflects on the principles here laid down, 

 there will arise no difficulty in explaining the circula- 

 tion in any particular case, if this plain precept be con- 

 stantly kept in mind : that, in consequence of the phys- 

 ical principle which has been assigned, a pressure will 

 always be exerted by the liquid which is ready to un- 

 dergo a change upon that which has already undergone 

 it a pressure which, as there is no force to resist it, will 

 always give rise to motion in a direction from the chang- 

 ing to the changed liquid. 



I then, in the work referred to and in my " Physiol- 

 ogy," continued the investigation to a determination of 

 the uses and action of the heart (heretofore considered 

 as the sole cause of the circulation), the action in as- 

 phyxia, the case of obstructed trachea, local inflamma- 

 tion, etc. 



By regarding the affinity between the blood and the 

 tissues with wl>ich it is in contact as the primary cause 

 of the circulation, we assign a reason for those various 

 phenomena which cannot be accounted for by Harvey's 

 doctrine: the motions in the embryo; the periodic and 

 local variations ; the portal circulation ; the changes in 



