182 



THE CIRCULATION OF THE BLOOD 



during the fall of blood pressure. The activity of the center produces a 

 slower rate of the heart during expiration, shown in figure 241. This vari- 

 ation in heart rate disappears when the vagi are cut off from the center. 



FIG. 181. Diagrammatic Representation of the Origin and Course of the Cardiac Nerves in 

 the Dog. Vag. Syn, Vago-sympathetic nerve; D 1 , D 6 , first to fifth dorsal spinal nerves. In- 

 hibitory fibers in red, accelerators in black. (Modified from Moret.) 



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 acceleration 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 



