294 COHN— STUDY OF THE [April 23. 



are found within it. With the estabhshment of the fact that the 

 auriculo-ventricular bundle possessed this mixed structure, discussion 

 as to whether conduction was nervous or muscular has gone out of 

 fashion. Its essential function consists in conducting impulses and 

 so coordinating the rhythms of the auricles and ventricles. To prove 

 that it actually performs this function, its structure has been de- 

 stroyed in experiments. In these the result anticipated was realized ; 

 auricles and ventricles continued to beat, each at a rate of its own 

 and each in a rhythm without reference to the other. Two other 

 facts are known about this bundle ; first, that it can conduct impulses 

 in a backward direction (from ventricles to auricles) as well as for- 

 wards ; and second, that it conducts a little slower, especially at its 

 upper terminal, than the rest of the heart muscle. We shall return 

 to a consideration of this structure in relation to electrocardiography. 



In 1906 another structure situated at the junction of the superior 

 vena cava and the right auricular appendix was discovered by Keith 

 and Flack. It is a small structure called the sino-auricular node. 

 It has a sectional area O'f 0.3 by o.i cm., and attains a length of from 

 2.0 to 3.0 cm. Its existence has been abundantly verified. Like the 

 conduction bundle, it also contains large and fine nerve elements. 

 The comparative anatomy of both these structures has been traced 

 by Ivy Mackenzie and by Kiilbs, while the embryology has been 

 studied by Professor Mall. It is the discovery of these two struc- 

 tures, the sino-auricular node and the auriculo-ventricular system, 

 which has added a second new and significant chapter to our store of 

 information relating to the mechanism of the heart-beat. 



The sino-auricular node has been recognized as the structure 

 which usually initiates impulses for the contraction of the whole 

 heart, and at the same time sets and maintains its rate. These prop- 

 erties have been attributed to it because excision or exclusion of the 

 node from function reduces the rate of the heart-beat, and some- 

 times even stops it. Afterwards it begins gradually to contract 

 again, but the original rate is not restored. It can usually be shown 

 that another portion of the heart now sets the pace. Other methods 

 have been employed to ascertain its functions ; warmth applied to the 

 site of the node accelerates the rate of the heart; cooling slows it. 

 Attempts to alter the temperature elsewhere of the surface do not 



