1892.] Physiology and PatJiology of the Mammalian Heart. 437 



On coming to the effect of the vagi upon the ventricles we find that 

 the distension of the heart during vagus actions is due to the ven- 

 tricles being more expanded, both in diastole and in systole. We point 

 out that the increased volume of the heart at the end of systole is a 

 necessary result of the increased contraction volume, and combat the 

 conclusions of those who ascribe it to weakening of the ventricular 

 contractions, pointing out that the greatly increased contraction 

 volume increases to a corresponding extent the work done at each 

 contraction. We give detailed reasons for concluding that this 

 suffices to explain the apparent diminution of the ventricular con- 

 tractions. 



We then examine the influence of the vagus upon the tonus of the 

 relaxed ventricles, and point out that the great distension during 

 vagus action is due entirely to increased intra-ventricular pressure 

 during diastole, and not, as has been asserted by some, to any change 

 in the elasticity of the relaxed ventricular wall. 



Next, we consider the cause of the rise of venous (systemic and 

 pulmonary) pressure, and find that this is due not to any increase in 

 the amount of blood entering the veins in a given time or to contrac- 

 tion of their walls, but that it is to be ascribed to the diminished 

 inflow into the ventricles. 



Th.3 cause of this diminished inflow into the ventricles leading to 

 corresponding diminution of the output is twofold, namely, weaken- 

 ing or arrest of the auricles, and, secondly, the elastic resistance of 

 the ventricular wall to distension. We show that this explanation 

 must apply to both sides of the heart, and that observed facts corre- 

 spond with it. 



We then consider the after-effects of vagus excitation, and show 

 that the temporary increase in the output which is sometimes present 

 may be explained by a temporary increase in the force of the auricular 

 contractions, and by the venous pressure taking some little time to 

 fall after the vagus excitation has ceased. 



After this, we examine the influence of the vagus upon the heart 

 rhythm, and show that, when the vagus excitation reaches a certain 

 degree (varying in different animals), the ventricles begin to beat 

 independently of the sinus and auricles ; that this rhythm, which is at 

 first slow and irregular, gradually becomes fairly rapid and almost 

 completely regular. 



This rhythm, we show, must be looked upon as the same as that 

 which, as Wooldridge and Tigerstedt observed, makes its appearance 

 when the ventricles are severed from the auricles. We point out, 

 however, that the independent ventricular rhythm of vagus action is 

 characterised by the slowness with which it establishes itself. 



This characteristic is due to the lowering of the excitability of the 

 ventricles produced by vagus action, and we adduce a considerable 



