204 THE CIRCULATION OF THE BLOOD 



The Accelerator Nerves. The influence of the accelerator nerves 

 reaching the heart through the sympathetic, is the reverse of that of the 

 vagus. Stimulation of the sympathetic, even of one side, produces acceler- 

 ation of the rate of the heart-beats, and, according to certain observers, section 

 of the nerve produces slowing. The acceleration produced by stimulation 

 of the sympathetic fibers is accompanied by increased force, and so the action 

 of the nerve is more properly termed augmentor. The sympathetic differs 

 from the vagus in several particulars other than the augmentation which it 

 produces; first, the stimulus required to produce any effect must be more 

 powerful than in the case with the vagus stimulation; second, a longer time 

 elapses before the effect is manifest; and third, the augmentation is followed 

 by exhaustion, the beats being after a time feeble and less frequent. The 

 stimulation of the vago-sympathetic in the frog, which usually produces 

 inhibition, will occasionally produce acceleration, especially if the heart is 

 beating feebly at the time of the stimulation. 



The fibers of the sympathetic system, which influence the heart-beat in 

 the frog, leave the spinal cord by the anterior root of the third spinal nerve. 

 They pass by the ramus communicans to the third sympathetic ganglion, 

 thence to the second ganglion, the annulus of Vieussens (around the sub- 

 clavian artery), through the first ganglion, and along the main trunk of 

 the sympathetic to near the exit of the vagus from the cranium. There 

 the two nerves join and run down to the heart within a common sheath, 

 forming the vago-sympathetic trunk. 



In the dog the augmentor fibers leave the cord by the anterior roots of 

 the second and third dorsal nerves, and possibly also by the first, fourth, 

 and fifth dorsal nerves. They pass by the rami communicantes to the gang- 

 lion stellatum, or first thoracic ganglion around the annulus of Vieussens 

 to the inferior cervical ganglion of the sympathetic. Fibers from the annulus 

 or from the inferior cervical ganglion proceed to the heart, figure 182. The 

 course of the augmentor fibers in the spinal cord is not so well known except 

 that they originate in an augmentor center in the medulla. The circulation 

 of venous blood appears to stimulate the augmentor center, and of highly 

 oxygenated blood the inhibitory center. 



The accelerator center, like the inhibitory, is in constant tonic activity; 

 and the cardiac acceleration on cutting the vagi, shown in figure 181, is in 

 part to be ascribed to this tone. When both nerves are stimulated together, 

 the resulting rate is the algebraic sum of the opposed influences, according 

 to Hunt. The accelerator center is influenced by afferent impulses arising 

 throughout the body, and these reflexes contribute to the general co-ordina- 

 tion of the chest with the activities of the body. 



In addition to direct and reflex stimulation, impulses passing down from 

 the cerebrum may have a similar effect. 



Other Influences which Affect the Heart. A great variety of special 



