MEMOIR XXVI.] CAPILLARY ATTRACTION, ETC. 373 



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 

 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 Ions; been recognized as bearing a strong; resem- 



o o o o 



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 which 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 



