270 PHYSIOLOGY. 



time after the irritation, then ensues a marked slowness. As to the 

 blood-pressure, irritation of the accelerators as a rule causes no change, 

 but instead of being elevated at times there is a fall. This lowering of 

 pressure is a result of the heart not being filled during the frequent 

 contractions. Hence the blood thrown out by the contractions of the 

 ventricles into the arteries is less, and blood-pressure falls. It was 

 formerly supposed that section of the accelerators did not change the 

 rhythm of the heart, but Eeid Hunt has shown that after extirpation of 

 the inferior cervical ganglion and the first thoracic on the two sides, 

 the vagi having been previously divided, the number of the heart-beats 

 diminishes. Vagus inhibition could be more readily produced after 

 the removal of the influence of the accelerators ; hence the accelerator 

 nerves, like the vagi, are always active in the normal heart. The 

 accelerators produce their effects much better when the heart is beating 

 moderately, for after the vagi are cut there is a considerable augmenta- 

 tion of the heart. 



The accelerators apparently, have less powerful functions, for 

 when the inhibitors and they are simultaneously irritated the effect 

 is inhibition. The phenomenon is less, however, than if the same 

 inhibitors had been stimulated by themselves. Aside from their great 

 and primary differences as to the effects produced, the accelerators 

 differ in that they require a greater intensity of stimulus to produce 

 any results; also in that a comparatively long latent period precedes 

 every effect. In every respect the accelerators seem to be directly 

 opposite to the inhibitors. They are the antagonists of the inhibitors. 



Cardio-accelerator Reflexes. H. E. Hering has shown by experi- 

 ments upon rabbits that the increased heart-beat by muscular exercise 

 does not ensue if the accelerator nerves have been divided, here a 

 cardio-accelerator reflex exists. Aulo arrives 'at the previous con- 

 clusion of Johansson that the increased heart-beat after muscular 

 exercise is neither due to chemical products of muscle activity nor to 

 peripheral irritation, but to an associated stimulation of the cardio- 

 accelerator center in the passage of a motor impulse. 



D. E. Hooker, after section of the vagi and slowing of the heart 

 by peripheral stimulation of one vagus, obtained acceleration of the 

 rate of the heart by irritation of the sciatic, splanchnic and other 

 nerves. All these facts indicate that the accelerator center stands in a 

 reflex relation to the peripheral nerves. 



Eeid Hunt believes that it is rather the cardio-inhibitory centers 

 which are inhibited than a reflex action exciting the cardio-accelerator 

 centers. 



