THE CIRCULATION OF THE BLOOD 



683 



node, together with the conducting function of the tissues in which the auriculo- 

 ventricular node is situated. The former is at the mouth of the superior vena 

 cava, and the latter at the opening of the coronary sinus, which are the 

 representatives, respectively, of the right and left ducts of Cuvier of the embryo. 

 Thus, the right and left vagi act preferentially, each on that structure with 

 which it would naturally be expected to be in morphological relationship. 



Colin and Lewig are of opinion that these results favour the view that the 



FIG. 233. ACTION OF ACCELERATOR NERVES IN- IMPROVING CONDUCTION. 



Upper curves, ventricle beats. Lower curves, auricle beats. 



1, Ventricle stimulated at the rate marked by the lowest signal, namely, 22 shocks in five seconds. 



The auricle does not follow until the accelerators are stimulated as shown by the top 

 signal. At the end of the tracing the artificial stimulation of the ventricle is stopped, 

 and the normal rate of the beat is seen (17 in five seconds). Time in seconds given by 

 the middle signal. 



2, At the beginning the natural rate of the heart beat is seen (14 in five seconds). During the 



period shown by the middle signal, the auricle was stimulated at a rate of 25 in five 

 seconds. The ventricle is unable to follow, until the accelerators are stimulated (top 

 signal). Time in seconds by bottom signal. 



(Bayliss and Starling, 1892, 2.) 



particular aspects of vagus action, obtained reflexly by Engelmann, depend on 

 the particular attributes of the tissue in which the fibres end, rather than on 

 different functions of the same muscle. 



Thos. Lewis (1914) further finds that the effect is more profound upon the 

 auriculo-ventricular node than upon the sino-auricular node. 



Some important conclusions as to the mode of action of the vagus on the 

 ventricle are to be drawn from the work of Mines (1914) on the frog's heart. 

 It is shown that atropine, applied to the sinus, eliminates the action of the vagus 



