THE HEART BEAT 165 



of tissue which resemble the tissue of the less developed or more 

 primitive heart. We find in the walls of the auricles small nodes 

 and islets of tissue, which no doubt represent the sinus tissues 

 found in the frog's heart. These nodes of tissue are really the 

 pacemakers of the heart, for it is in them that the impulse or 

 stimulus arises which sets agoing the contraction of the auricles 

 and the ventricles. These nodes are connected by fibers with the 

 musculature of the auricles and ventricles, those running from 



Fig. 18. Dissection of heart to show auriculo-ventricular bundle (Keith) ; 

 3, the beginning: of the bundle, known as the A-V node ; 2, the bundle dividing 

 into two branches ; 4, the branch running on the right side of the interven- 

 tricular septum. (From Howell's Physiology.) 



the auricles to the ventricles being gathered into a bundle of tis- 

 sue which has been named the bundle of His (Fig. 18). 



Numerous cases have been recorded of individuals having a 

 very irregular or a very slow heart beat in whom post-mortem 

 examination of the heart showed a diseased condition of the 

 bundle of His. The conditions observed in man have been re- 

 produced in the case of animals by cutting or clamping the tis- 

 sue about this bundle. The result is much the same as that ob- 

 served in the turtle's heart when the string is tied between the 

 auricle and the ventricle. The ventricle may continue to beat, 

 but it does so without reference to the auricles. Such a condi- 

 tion is known as heart Hock. 



