ACTION OF THE INHIBITOR Y NER VES. 



205 



Excitation Siqna.1 



Time in Seconds 



t^jwvyvr 



are in marked contrast with what is seen when the vagus is stimulated 

 in the ordinary manner. In the first place, I have been able to obtain 

 absolute standstill with a strength of current immensely weaker than 

 what is required to obtain any effect when the vago-augmentor nerve 

 is stimulated. In one case the strength of current was as weak as is 

 just necessary to cause contraction of a striated muscle upon stimulation 

 of its motor nerve, the secondary coil being at 35 cms. from the primary 

 with one Daniell cell ; at the distance of 12 cms. from the primary it was 

 just perceptible to the tongue. In the second place, it is astonishing how 

 long the standstill sometimes lasts after the stimulation is over. Thus 

 one of my tracings shows that a stimulation of the intracranial vagus, lasting 

 thirty-eight seconds, caused the suspended heart to remain absolutely still 

 for 290 seconds, i.e. for 252 seconds after the end of the stimulation. In 

 the third place, it is possible to keep the heart absolutely still by means 

 of continued stimulation 

 for a very much longer 

 time than is possible when 

 the vagus is stimulated 

 in the ordinary manner. 

 Thus, as I mentioned at 

 Copenhagen, I have kept 

 the heart absolutely qui- 

 escent for as long a time 

 as twenty-eight minutes 

 by continued stimulation 

 of the intracranial vagus 

 with a weak stimulus (sec. 

 coil between 12 and 11 

 cms. from primary). 



In order to investigate 

 more fully the action of 

 the inhibitory fibres alone, 

 I turned my attention to 

 the land-tortoise (Testudo 

 grceca) an animal which 

 not only has the advantage 

 that its inhibitory nerve 

 is separate from its accel- 

 erator, but also possesses, 



as already described, a free coronary nerve, the fibres of which belong to 

 the vagus system. It is possible to cut this nerve and stimulate it without 

 in any way damaging the heart, and so an insight can be obtained into 

 the action of the inter-auricular nerves in an intact heart. The presence 

 of this free nerve, between the sinus and auriculo-ventricular junction, 

 has proved invaluable in increasing our knowledge of the action of the 

 inhibitory nerves. Fig. 114 shows the effect of stimulation of the isolated 

 coronary nerve, the sinus end having been cut and ligatured. If Fig. 

 114 be compared with Fig. 115, which shows the effect of stimulation 

 of the right vagus in the neck, we see that the difference between the 

 two curves consists simply in this, that the coronary nerve diminishes 

 the strength of the contractions of the auricle without affecting the rate 

 of the beat, while the vagus diminishes both the rate and the strength of 

 the auricular contractions. In this instance the slowing of the rate of 



Ventricle 



Fig. 109. — Heart of toad. Suspension method. Clamp 

 in auriculo-ventricular groove. Short stimulation 

 of the intracranial vagus roots with current of 

 moderate strength. The change of level in the 

 auricular tracing during the stimulation is due to 

 the movement of the laryngeal and trapezius 

 muscles. 



