1888.] Arterial Pressure on the Mammalian Heart. 289 



the descending aorta do not run parallel with those occurring in the 

 auricles. The ventricles, while they beat more slowly than before, 

 usually beat much more strongly even when the auricular contraction 

 has become markedly weakened (fig. 1). Depression of the ventricular 

 force may occur, but it comes considerably later than the auricular 

 depression, and is very much slighter in degree (fig. 3) . The slow 

 strong ventricular systoles are able to empty the cavity of the left 

 ventricle when systoles of less strength fail to do so — as indicated by 

 the fact that the recording lever often fails to descend to the ordinary 

 level in the interval between the contractions (fig. 1). When the 

 descending aorta has been released and the pressure has fallen, a 

 period of marked cardiac acceleration often succeeds ; during this 

 acceleration, the individual ventricular beats are much diminished in 

 force (fig. 3). 



The above-mentioned cardiac changes attendant on a sudden rise of 

 arterial pressure are brought about through the medullary cardio- 

 inhibitory centre and the vagus nerves. They are of such a nature 

 that while the ventricles are contracting slowly and powerfully in 

 such a way as to be able to discharge their contents in spite of the 

 increased arterial pressure, there occurs a striking change in the 

 action of the auricles involving a great reduction in the amount of 

 blood pumped into the ventricles and the degree in which the latter 

 are distended just before their systole. Hence the quantity of blood 

 thrown out by the left ventricle into the systemic arteries is much 

 diminished, and the rise in the blood-pressure is in some measure 

 counteracted and controlled. 



II. In conditions where the medullary cardio -inhibitory mechanism 

 has ceased to exert any controlling influence upon the heart (e.g., 

 after section of both vagi), the effects following a sudden rise of 

 arterial pressure are entirely different from those above described, 



Marey showed that there was no very constant relation between 

 the rate of the heart's action and the height of the blood-pressure 

 after section of the vagi ; some degree of acceleration was commonly 

 observed. 



Examining the cardiac changes in the way already mentioned, I 

 find that after section of the vagi or paralysis of the medullary 

 cardio-inhibitory mechanism, a sudden rise of arterial pressure causes 

 no very striking or constant change in the heart's rhythm ; frequently 

 there is slight acceleration. There is a complete absence of the cha- 

 racteristic changes in the contraction force above described (under I). 

 As regards the strength and character of the cardiac action, there are 

 two conditions to be noted. 



(1.) The heart may at each systole be able to discharge its contents 

 in normal or approximately normal fashion. Tn such circumstances 

 the principal change to be observed in a vigorous heart is a marked 



