564 



Mr. T. Lewis. 



the mouths of the two cavae ; it extends along the ventral border of the 

 superior cava for some distance. This discovery was significant and prompted 

 the experiment which I am about to describe. Simultaneously with the 

 observations of my laboratory, Wybauw, in Li^ge, instituted independent 

 observations (14 and 26). The results of these investigations were in perfect 

 harmony ; we speedily found that the region of the sino-auricular node 

 becomes relatively negative before any othei- point on the surface of the 

 auricle. The method first adopted was relatively crude ; we sought the 

 direction of the first flow of current produced by the action of the auricle 

 through the galvanometer, when leading off from pairs of chosen points on 

 the auricular surface. It was shown that, providing one contact lies over 

 the sino-auricular node, it is a matter of indifference where the second 

 contact is placed, the S-A contact is always negative at the start of the 

 electric change. These observations have since been abundantly confirmed 

 by similar observations and by the method of cooling. The last method 

 merits further description. A withdrawal of heat from the tissues is well 

 known to depress their functions, and in experiments upon the frog's heart, 

 cooling of the sinus region was known to decrease the rate of the heart's 

 beating. It does so because it depresses the function of the tissue elements 

 in which the heart's rhythm has its origin. MacWilliam (19) and Mack (7) 

 were responsible for some of the earliest experiments on cooling in the 

 mammalian heart. The rate of the mammalian heart beat is lowered when 

 the area containing the sino-auricular node is cooled, and this reduction of 

 rate is provoked by cooling no other region of the heart (8). This experi- 

 ment has confirmed in a striking manner what we already knew, namely, 

 that the wave of contraction starts in this locality, but it has shown further 

 that the impulses which promote the rhythm are born there. 



At a later date a method (16) was devised in my laboratory whereby the 

 time at which the excitation wave reaches any given point on the surface or 

 lining of the auricle can be determined with a maximal error of 0"002 second. 

 This method allowed us to map out in a precise fashion the wave of excita- 

 tion in the auricular musculature over the greater part of its course. Our 

 conclusions may be briefly summarised. The excitation wave starts in the 

 head or swollen part of the sino-auricular node ; it spreads from this node at 

 an average rate of 1000 mm. per second into the surrounding auricle along 

 lines radiating in every direction from the node. It is noteworthy that the 

 surrounding muscle is arranged in bands which converge in this region of the 

 heart as though arranged to speed the spread. I have likened the spread in 

 the auricle to the spread of a viscous fluid poured from a funnel upon an 

 almost fiat surface ; the margin of the fluid invades the plate as an ever 



