186 



THE CIRCULATION OF THE BLOOD 



longest time after death. Although, the observation does not enable us 

 to determine exactly where the heartbeat originates, yet it makes it 

 very probable that this is somewhere in the auricles ; a conclusion which 

 is borne out by many other pieces of evidence, such as those obtained by 



Fig. 51. Diagram of an auricle showing the arrangement of the muscle bands; the concen- 

 tration point (C.P.); and the outline of the S.A. node (S.A.N.). The diagram is to scale, and 

 illustrates by the circles and connecting dotted lines the method of leading off by paired contacts 

 and the subsequent orientation. (From Thomas Lewis.) 



I Auricular appendage 



HR- 



node 



-Auriculoventricular node 

 -Aurlculoventricular bundle 



Right & left ventricular 

 * bundles 



-Musculi papillares 



Fig. 52. Diagram to show the general ramifications of the conducting tissue in the heart of 

 the mammal. It will be observed that there is none of this tissue between the sinoauriculo- and 

 auriculoventricular nodes. 



the study of polysphygmograms (page 285), of electrocardiograms and 

 of observations on the heart during heart-block (page 270). Our problem 

 therefore narrows itself down to determining the exact point of the right 

 auricle at which the beat originates. 



