CIRCULATION OF THE BLOOD. 551 



will be likewise arrested in diastole. If the mesenteric nerve be stimu- 

 lated by an induction current the heart will also be brought to a sudden 

 standstill. If both pneumogastric nerves be divided, the anterior roots 

 of both spinal accessory nerves be torn out, or the medulla oblongata 

 destroyed, the preceding experiments will all fail. This indicates that in 

 the first case the impulse was conducted through the central stump of 

 the divided pneumogastric to the brain, and there, from a collection of 

 nerve-cells in the medulla oblongata, which is termed the cardio-inhibi- 

 tory centre, is transmitted through the spinal accessory nerve to the 

 pneumogastric plexus, and through the undivided vagus to the heart. 

 If the spinal accessory nerve be pulled out by the roots, the eardio- 

 inhibitory fibres of the pneumogastric undergo degeneration, and four or 

 five days after the operation stimulation of the vagus fails to slow the 

 heart. In the other instance, where irritation of the intestinal surface 

 or mesenteric nerves produces inhibition, the impulse is conducted 

 through the mesenteric and sympathetic chain to the spinal cord, from 

 there to the cardio-inhibitory centre, and from there to the heart. It is 

 probable that the syncope occasionally produced by severe pain, emotions, 

 or sometimes from drinking ice-water when overheated is produced in 

 the same manner, the heart being arrested or slowed through reflex 

 inhibition. 



It is thus observed that the pneumogastric nerve possesses a function 

 directly opposed to that of most other nerves. When a motor nerve is 

 stimulated it produces contraction of the muscles to which it is dis- 

 tributed. Stimulation of the pneumogastric, on the other hand, produces 

 relaxation of the heart-muscle. The manner by which this effect is 

 produced is in all probability through the inhibition of the motor ganglia 

 of the heart. 



The cardio-inhibitory centre in the medulla is in constant action, 

 through reflex stimuli conducted to it through the abdominal and cervical 

 sympathetic, and might be compared to the action of a brake on a 

 moving machine. When both pneumogastric nerves are divided, the 

 heart in the dog, and to a less extent in the rabbit, beats very much 

 faster, so that this inhibitory centre is constantly restraining the a-ctiou 

 of the motor ganglia. The cardio-inhibitory centre may be directly 

 stimulated by sudden anaemia of the medulla, as by sudden ligation of 

 both carotids; by sudden venous hyperaemia, as by ligation of all the 

 veins of the neck; or by increased venosity of the blood, as by dyspnoea 

 or causing an animal to inhale COj. It may also be reflexly stimulated, 

 as already indicated. 



The Accelerator Nerves of the Heart. — The action of the heart 

 may not only be retarded or arrested through the stimulation of the 

 pneumogastric, but in the mammal, even after division of both pneumo- 



