224 



THE CIRCULATION OF THE BLOOD 



which, it will be remembered, dominates the beat in the rest of the 

 heart. There is evidence, however, that the vagus also directly affects 

 the rhythm of the ventricle. It may be stated as a general conclusion 

 from these results that the influence of the vagus upon the heartbeat is 

 chiefly centered upon those parts of the organ in ivhich the rhythmic power 

 is most highly developed. 



Besides affecting the rate and strength of the heartbeat, the vagus also 

 exercises a control on the conductivity of the cardiac muscle. Thus, if 

 a partial block is instituted in the turtle heart by applying a clamp be- 

 tween the auricles and ventricles, stimulation of the vagus enfeebles the 

 auricular beat and may also cause a complete heart-block as shown in 

 the tracing reproduced in Fig. 66. It is important to point out here, 

 however, that under certain conditions the vagus may appear to increase 

 rather than decrease the conductivity of the tissue in the auriculoven- 



Fig. 66. Tracing to show that vagus stimulation may diminish transmission from auricles to 

 ventricles. It shows the effect of stimulating the left vagus on partial (2/1) block produced on 

 heart of turtle by application of clamp at auriculoventricular junction. Stimulation at ^ depressed 

 the conductivity and weakened the auricular contractions (lower tracing) without slowing their 

 rate. The result was an increase in the degree of block with cessation of ventricular contractions 

 (upper tracing). Initial auricular rate = 35 per minute. (From Carrey.) 



tricular junction; for example, it has been observed in the turtle heart 

 that when a clamp is so tight as to produce complete block that is to 

 say, to render the ventricle inactive while the auricle is still beating at 

 the usual rate stimulation of the vagus, besides causing the auricles to 

 become distinctly slowed, may at the same time cause the ventricles to 

 respond to the auricular beats. This result is probably due to the better 

 chances of slow beats getting through the junction than those which are so 

 frequent as to crowd one another on the narrow bridge which the con- 

 stricted tissue offers to their passage (Fig. 67). 



Very important work was contributed in this field by G. R. Mines 13 

 shortly before his lamentable death. He found that the local applica- 

 tion of atropine to the sinus eliminates the effect of stimulation of the 

 (intracranial) vagus on the rate of the heartbeat, while the effect on the 



