OF VITAL MOTION. 121 



this is not all that concerns the action of the blood 

 upon the heart : and on further examination we may 

 find, in the same quarter, a reason why the systole 

 should follow the diastole. The jet of blood in the 

 coronary arteries is withheld, as we know, in con- 

 sequence of the diastole, and hence the arterial tension 

 must be less perfect. The blood, also, at the same 

 time is passing, or has already passed, into the veins, 

 and become deprived of its arterial character. It 

 has in fact given up, in great measure, the principle 

 which is the main stimulus of activity and life, 

 namely, the oxygen ; and therefore it is less fitted to 

 sustain the diastole, than it was when freshly injected 

 into the vessels. The arterial tension and capillary 

 action are in fact reduced, if not removed ; and hence 

 we may conclude that as this tension and action was 

 a cause of the diastole, so the removal will tend to 

 bring about the systole, by allowing, that is to say, 

 the heart to return to its former condition of 

 contraction. 



To this explanation it can scarcely be objected, that 

 there is no intermittent variation in the quantity of 

 blood contained in the walls of the heart. At each 

 systole the blood is expelled from the cavities, and we 

 can hardly suppose that this contraction should take 

 place without expelling some of the blood in the 

 walls as well. Indeed, if we watch the action of the 

 heart in a frog, in which the sternum has been 



