A CTION OF THE INHIBITOR Y NER FES. 209 



simultaneously with the muscle, whenever the muscle is directly 

 stimulated. The interpretation, in fact, which Schiff gave to the 

 local diastole, caused by direct stimulation of the frog's ventricle, is 

 confirmed and elucidated by these experiments on the behaviour of the 

 strip of tortoise auricle to direct stimulation. 



One of the most striking primary depressing effects of stimulation of 

 the vagus nerve is the depression of conductivity. In the intact heart 

 such depression takes place at the two natural blocking points, namely, 

 the sino-auricular and auriculo-ventricular junctions ; in my experiments 

 with the suspended heart I have frequently obtained upon stimulation 

 of the vagus, especially in the case of the crocodile, tortoise, and snake, 

 an absolute standstill of both auricle and ventricle, although the sinus 

 has continued to beat regularly. In these cases it was perfectly clear 

 upon inspection that the contractions of the sinus stopped absolutely at 

 the sino-auricular junction, and did not pass into the auricle. Fig. 115 

 affords an example of this phenomenon. 



So also in the case of the suspended heart of the frog, with the 

 clamp at the auriculo-ventricular junction, I have seen instances where 

 stimulation of the vagus nerve has caused the ventricle during the 

 stimulation to respond to only every second auricular contraction. In 

 such cases, owing to the simultaneous stimulation of the accelerator 

 nerve fibres, the rate of rhythm was not slowed, and in consequence the 

 diminution in conductivity was able to be manifested. 



This depression of conductivity at the auriculo-ventricular junction, as the 

 result of stimuli of inhibitory fibres, gives a simple explanation of Eckhard's 

 experiment, 1 that stimulation of the cut septal nerves in the frog causes the 

 ventricle to stand still, although both auricle and sinus continue beating. 



Perhaps the clearest evidence of the primary influence of the vagus 

 nerve upon the conductivity of the auricular muscle is given by 

 stimulation of the nerve when the auricle of the tortoise is slit up 

 so as to cause a partial block in the passage of the contraction wave 

 over the two parts of the auricle as in Fig. 105. In this case for a 

 long time the contractions of Av and V follow only upon every second 

 contraction of As, and the result of stimulation of the inhibitory fibres 

 supplying the auricle is to make the block complete, so that during the 

 stimulation no contractions pass from As to Av, and the ventricle 

 therefore remains still. Naturally this experiment succeeds only when 

 stimulation of the vagus does not cause a standstill of the whole heart, 

 and is therefore most easily seen upon stimulation of the coronary nerve, 

 rather than of the vagus nerve itself. 



It is clear from what has been said, that apparent standstill of the 

 heart, i.e. of the auricle and ventricle, may be produced by stimulation 

 of the vagus in three distinct ways — 



1. By the depression of rate in the sinus. 



2. By the depression of conductivity at the sino-auricular junction, 

 so that the sinus continues beating, but the auricle and ventricle remain 

 still. 



•">. By the depression of the force of the contraction to invisibility, 

 so that, although the contractions pass from the sinus into the auricle, 

 these latter are so diminished in force as to be not apparent. 



Such a depression of the force of the contractions of both auricle 

 1 Bcitr. z. Anaf. v. Physiol. {Eckha/rd), Gieasen, 1876, Bd. vii. 

 VOL II. — 14 



