Upon the Motion of the Mammalian Heart. 



563 



This instrument is the string galvanometer, and its construction must rank 

 as a chief milestone in the study of the questions which we are considering. 

 It has been known for very many years that, when muscle becomes active, 

 that is to say, when it passes to a state immediately premonitory to the 

 actual contraction, it becomes relatively negative to inactive muscle, in the 

 sense that the zinc element of a copper-zinc couple is relatively negative to 

 the copper element. When a wave of contraction passes through a strip of 

 muscle, it travels in the immediate wake of what is termed the excitation 

 wave, a wave of electrical change whose crest is a crest of relative 

 negativity. Each part of the tissue, as it is about to become involved in 

 the contraction process, shows this electrical change ; the moment at which 

 each part of the tissue is about to contract can be signalled by a sufficiently 

 delicate instrument. It was this fact and Einthoven's instrument of which I 

 took advantage when I commenced my studies of the origin and propagation 

 of the contraction wave throughout the heart. 



From this general historical introduction, we may proceed to examine the 

 chief problems which presented themselves, and which have now been solved. 



The Pacemaker of the Mammalian Heart and the Spread of the Excitation 



Wave in the Auricle. 



In the cold-blooded heart the contraction wave has been recognised long 

 since to start in the region of the mouths of the great veins which enter the 

 sinus venosus, for in the frog and the tortoise the sinus contracts first, and is 

 followed by contraction of the auricle. This sequence may be witnessed in 

 the frog or tortoise with the unaided senses ; it is disturbed if a clamp is 

 placed upon the sinus, for then, while the mouths of the veins continue to 

 beat at their old rate, the whole of the auricle and ventricle remains 

 quiescent. 



In the mammalian heart an anatomical sinus does not exist, sinus and 

 auricle have become closely incorporated ; but morphological reasoning 

 directed attention to the mouths of the great veins, to the superior and 

 inferior vena cava, as the probable starting point of the contraction wave. 

 A number of early experiments, conducted for the most part upon the dying 

 heart or upon the heart mutilated by fatal incisions, purported to demon- 

 strate that the heart _beat starts in the region of the great veins ; this 

 localisation lacked both precision and certainty. A chief step (12) was taken 

 when Keith and Flack (1907), in their search for sinus remnants, lighted 

 upon the highly differentiated mass of neuro-muscular tissue, which has 

 since been termed the sino-auricular node. This mass of tissue lies in the 

 dog immediately to the caval side of the sulcus terminalis, a line bounding 



