OF VITAL MOTION. 123 



arterial; and unless we had some common cause in 

 operation (as we have in the general distribution of 

 new blood through the coronaries) it is difficult to 

 account for the simultaneous occurrence of diastole 

 and systole in these two sides. It is an assumption 

 without any proof, that the right side of the heart is 

 more sensitive than the left, and that, by reason of 

 this, it is able to respond to the feebler stimulus of 

 venous blood : indeed this assumption is in opposition 

 to the known law, that the acuteness of sensibility is 

 in direct relation to the arterialized condition of the 

 blood. We may conclude, therefore, that the action 

 of the blood in exciting the heart, is exerted upon 

 the vessels and fibres of the walls of the organ (and 

 perhaps in certain additional reactions in the vessels 

 elsewhere), rather upon the lining membrane of the 

 cavities, although we are not prevented from sup- 

 posing that there is a certain and definite action of 

 the same kind in the latter quarter. 



In the action of the heart, therefore, we detect the 

 harmonious co-operation of extra-organic and intra- 

 organic agents, with which we are already familiar, 

 but the theory is not complete except it will account 

 for the presence of the rhythm in the heart, and the 

 absence in all other muscular structures. Why it is 

 present in the heart we may find on examination. The 

 valvular construction of the organ may be one reason, 

 for this necessitates a continual admission of fresh 



