450 



THE DYNAMICS OF CARDIAC ACTION. 



ence with other nerve-impulses), of the functional activity of 

 an organ. Indeed, direct "inhibition" does not appear to us 

 to prevail in the organism any more than does direct "vaso- 

 dilation." NOT do we find the need of such an action to account 

 for the "inhibition" phenomena witnessed in connection with 

 the heart provided, of course, the functions we ascribe to the 

 vagus are considered as factors of the process through which 

 this organ is inhibited. It seems evident to us that, if this 

 nerve can quicken or moderate the pace of the cardiac contrac- 

 tions, arrest of these contractions must ensue if the impulses 

 transmitted through the nerve exceed in vibratory rhythm the 

 maximum rhythm of which the heart-muscle is capable. In 

 other words, there must obviously be a limit to the speed with 

 which the cardiac muscle can repeat its contractions within a 

 given time, and, excessive stimulation involving a greater num- 

 ber of heart-beats than the muscle can satisfy, cessation of the 

 organ's work normally follows. 



"If while the beats of the heart of a frog are being care- 

 fully registered," says Professor Foster, "an interrupted cur- 

 rent of moderate strength be sent through one of the vagi, the 

 heart is seen to stop beating. It remains for a time in diastole 

 perfectly motionless and flaccid; all the muscular fibers of the 

 several chambers are, for the time being, in a state of relaxation. 

 The heart has been inhibited . . ." The overwhelming im- 

 portance of this simple experiment becomes emphasized when 

 the influence of excessive suprarenal activity upon the oxida- 

 tion processes of the vagal centers, to which we have just re- 

 ferred, are given their full meaning. Indeed, the cardiac arrest 

 that follows over stimulation of the adrenals under the influence of 

 toxics is due to excessive activity of the oxidation processes in a 

 certain proportion of cases. 18 



Whether cardiac inhibition be due to cessation of supra- 

 renal functions or to excessive vagal stimulation, the effect on 

 the heart is the same as regards the mode of arrest: i.e., in 

 diastole. We have just seen that it occurs in the frog when 

 the vagus is stimulated. We have also submitted evidence to 



18 As such cases are clearly defined, as we shall see in a subsequent chapter, 

 their importance lies in their differentiation from cases in which the adrenals 

 are primarily arrested through excessive stimulation of their center: the an- 

 terior pituitary body. 



