1914-] MECHANISM OF THE HEART-BEAT. 295 



have this effect. But more important information still is gained by 

 means of the galvanometer. It has been demonstrated that each 

 heart-beat begins at the node, because artificial contractions due to 

 stimuli applied here yield curves of a shape identical with those re- 

 sulting from spontaneous discharges; similar stimuli applied else- 

 where fail of this identity. And finally, the law that the site at which 

 contraction begins is primarily negative to other portions of the same 

 strip of muscle is valid here. Lewis and Oppenheimer, Wybauw, 

 Clement and Sulze have all been able to show that in contraction, 

 the site of the node is primarily negative to all other portions of the 

 auricular surface. 



We have described the newly discovered structures in the mam- 

 malian heart. The function of the sinus node, in so far as it is now 

 known, is to initiate impulses for the whole heart and to determine 

 their rate; Lewis has aptly called it the pacemaker. The auriculo- 

 ventricular bundle provides for coordination in the complicated 

 mechanism of the heart. We must next show how, in the light of 

 these structures, the electrocardiogram has been employed in eluci- 

 dating the mechanism of the heart-beat. Althought the sinus node 

 and the conduction bundle are very small indeed in comparison with 

 the size of the whole heart, it is chiefly to these that the attention of 

 investigators has been directed, while to the great mass of the organ 

 which is charged with the real work of carrying on the circulation, 

 very little research has been devoted. Our account of the electro- 

 physiology of the heart must, therefore, be necessarily incomplete, 

 and incomplete in just the direction in which one had hoped for light, 

 — namely, in an attempt to employ electrical estimations as measures 

 of the contractile force of the heart. 



To be useful, the first demand of a method is that it give constant 

 readings, and observation has shown that the electrocardiogram satis- 

 fies this demand, for its waves tend to remain unaltered in shape and 

 size. When they alter, an ascertainable disturbance has in many 

 cases been found as the cause. An electrocardiogram is, therefore, 

 a reliable record. Its constancy is illustrated in records obtained 

 from various classes of animals. They have certain characteristics 

 in common, so that one can easily distinguish, for example, electro- 

 cardiograms of amphibia from those of the higher mammals. And 



