THE CARDIAC NERVES. 



523 



ganglion cells in the first thoracic ganglion, while others apparently 

 make their first termination in the inferior cervical ganglion. The 

 accelerator fibers may be stimulated in the spinal roots in which 

 they emerge (II, III, IV), in the annulus or in some of the branches 

 that arise from the annulus or from the inferior cervical ganglion 

 (5, 3, 2). It will be borne 

 in mind that no accelerator 

 fibers are found in the cer- 

 vical sympathetic above the 

 inferior cervical ganglion. 



At various times investiga- 

 tors have asserted that accel- 

 erator fibers are contained also 

 in the vagus nerve. Thus, it 

 has been shown that, after the 

 paralysis of the inhibitory fibers 

 in the heart by atropin, stimula- 

 tion of the vagus causes an accel- 

 eration of the heart. Little at- 

 tention has been paid to the 

 physiology of these fibers, since it 

 seems evident that the great 

 outflow of accelerators is made 

 via the sympathetic system. 



The Action of the Ac- 

 celerator Fibers. In ex- 

 perimental work the accel- 

 erators are usually stimu- 

 lated in one or more of the 

 branches represented sche- 

 matically as 5, 3, 2, in Fig. 

 216, or in the annulus. The 

 effect is an increase in the 

 rate of beat of the heart, 

 which may be very evident, 

 amounting to as much as 

 70 per cent, or more of the 

 original rate, or may be very 

 slight. When acceleration is 

 obtained the latent period is 

 considerable and the heart 



does not return at once to its normal rate upon cessation of the 

 stimulus (see Fig. 217). In some cases, perhaps in most cases, the 

 effect upon the heart is an acceleration pure and simple, that is, 

 the rate of beat is increased without any evidence of an increase in 

 the force of the beats. The larger number of beats is offset by the 

 smaller amplitude of each beat; so that the blood-pressure in the 

 arteries is unchanged. In other cases the effect upon the heart may 



in 

 w 



Fig. 216. Schematic representation of the 

 course of the accelerator fibers to the dog's heart 

 right side. (Modified from Pawlow.) The sym- 

 pathetic nerve is represented in solid black. The 

 course of the accelerator fibers is indicated by ar- 

 rows. /, Cervical sympathetic combined in neck 

 with, 10, the vagus; 77, 777, IV, rami communi- 

 cantes from the second, third, and fourth thoracic 

 spinal nerves, carrying most of the accelerator fi- 

 bers to the sympathetic chain ; 7, annulus of Vieus- 

 sens; 8, inferior cervical ganglion; 2, 3, 4, 5, 

 branches from vagus and vago-sym pathetic trunk 

 going to cardiac plexus (some of these 3, 5, 

 carry accelerator fibers; 9, the inferior laryngeal 

 nerve. 



