124 Dr. J. A. McWilliam. The Structure and Rhythm [Jan. 29, 



heart seems (from a comparison of the results obtained by direct 

 stimulation in the normal and inhibited states) to be unchanged. A 

 contraction can be elicited by a direct stimulus applied during the 

 inhibitory standstill quite as readily as in the normal condition of the 

 heart. 



Upon the contraction force of the ventricular muscle, stimulation 

 of the vagus nerve seems to be entirely without effect. 



Ventricle tracing showing inhibition resulting from stimulation of vagus nerve. 

 The recommencing beats are seen to be large. 



When a heart which is beating at a fairly rapid rate is slowed by 

 very gentle vagal stimulation, the ventricular beats are larger pro- 

 portionately to the slowing — up to a certain maximum. The increase 

 in the force of the beats is dependent on the slowing ; it occurs when 

 the heart is slowed by means other than by vagal influence, e.g., by 

 the application of cold to the sinus. It is the converse of what 

 occurs during rapid action of the heart, such as may result from the 

 application of heat to the sinus, or may follow vagal inhibition as an 

 after effect. During rapid action of the heart the ventricular beats 

 become smaller in proportion to the increased rates, until at length, if 

 the acceleration is excessive, the recording lever writes a mere wavy 

 line on the smoked paper. 



When the heart is arrested by vagus stimulation, and the ventricle 

 is made to contract by a direct stimulus, the force of the beat so elicited 

 is, even during the most powerful inhibition, quite as large as the 

 ordinary beats occurring spontaneously before the standstill. Indeed 

 an artificial beat elicited during inhibition is generally larger than 

 the spontaneous beats occurring before inhibition, just as the beats 

 occurring during a very slow cardiac rhythm are larger than those 

 seen during a quicker rhythm. A rapid artificial rhythm induced in 

 the ventricle during inhibition of the heart shows a diminution in the 

 size of the rapidly recurring beats, corresponding to what is seen in a 

 rapid natural rhythm. 



