THE HEART 103 



after which they spread over the ventricles. At times the 

 contraction may originate in the ventricle. Thus by drawing 

 a tight ligature about the heart at the junction of the auricles 

 and ventricles, the rhythm of the heart is disturbed and the 

 ventricle beats with an independent slower rhythm. If the 

 electrical changes of the beating heart are investigated, it is 

 found that the base becomes negative before the apex, and 

 that this condition of negative potential passes along in the 

 form of a wave to the apex. Its speed has been found to aver- 

 age at least 50 mm. a second. The latent period of frog's heart 

 muscle is about 0.08 of a second, but the change of potential 

 takes place instantly after the application of the stimulus. 

 The excitation wave can be made to pass over the heart in any 

 direction, and the speed with which it , travels indicates that 

 it passes through muscle and not through nerve. The duration 

 of the pause or block in the frog's heart has been found to be 

 from 0.15 to 0.30 of a second. The speed of the excitation 

 wave in embryonic muscle (3 to 11 meters a second) makes it 

 plausible that in the heart the block is due to the fact that 

 the excitation wave is transmitted through embryonic muscle 

 fibers that exist between the auricle and ventricle. The con- 

 nection between auricles and ventricles in mammals is made 

 by a strand known as the atrioventricular bundle. It begins 

 in the auricular septum as a definite bundle. Branches on the 

 auricular side may be traced back as far as the great veins 

 or into the coronary sinus. This bundle passes down to the 

 ventricular septum where it divides, one limb passing into 

 the right ventricle beneath the endocardium and one into the 

 left ventricle. Within the ventricles each limb forms a branch- 

 ing system long known as the Purkinje cells. There is no 

 doubt that it conducts the wave of contraction or excitation 

 from auricles to ventricles, for compression of the band pro- 

 duces a block. With a certain degree of pressure the ventricles 

 beat only once for each two beats of the auricle. With greater 

 pressure only once for each three or more auricular beats until 

 finally, the ventricle beats independently of the auricles with 

 a slower rhythm. In disease such an interference is not un- 

 common and is known as Stokes-Adams disease. The ventricle 

 then beats about thirty per minute and this rate is but little 

 influenced by events, like mental excitement, which normally 



