294 THE CIRCULATION 



the nervous system only controls its beat, coordinating it with the needs 

 of the organism. The recent researches of Dogiel and his colleagues 

 indicate very strongly that neurones centering in the various ganglia 

 of the heart itself direct and perhaps initiate the actual contractions of 

 the organ. These knots of short neurones are in the closest relation with 

 the central nervous system, and it is with the latter and the nerves be- 

 tween it and the heart that we are chiefly concerned. (See the experi- 

 ments in the Appendix, page 515.) On the other hand, the auriculo- 

 ventricular muscle-bundle, which connects intimately and directly the 

 inter-auricular septum with the musculature of the ventricles tends 

 to complicate still further the question of heart-actuation. The large 

 number of facts and suppositions which have been accumulated on each 

 side of this far-reaching question of the relation of muscle and nerve 

 remain to be unified into the certain truth. 



In general terms the nervous impulses connected with the heart appear 

 to be at least of three sorts. In the first place, there is probably a set of 

 afferent nerve-paths (in the rabbit and dog called the depressor) which 

 keep the central nervous system informed as to the nutritive and hydraulic 

 conditions in the heart-muscle. There are influences always coming 

 to the heart which tend to check its action namely, the inhibitory influ- 

 ences. And there are impulses continually passing to the heart which 

 cause it to augment its activity. 



THE SYMPATHETIC INFLUENCE (to consider the last first) comes 

 probably to the muscle-cells of the heart from a center in the medulla by 

 a route now fairly well known in the dog, rabbit, and cat; it is doubtless 

 similar in man. The fibers bearing these augmentor impulses leave the 

 spinal cord by the anterior roots of the second and third, and perhaps 

 fourth and fifth, thoracic nerves, pass, by means of the rami communi- 

 cantes, to the ganglion stellatum (the first thoracic ganglion of the sym- 

 pathetic), thence upward through the annulus of Vieussens (surrounding 

 the subclavian artery) to the inferior cervical ganglion. Thence, and 

 from the annulus as well, non-medullated fibers pass to the heart-muscle 

 by way of nerve-cells. These fibers are post-ganglionic branches 

 (axones) of cells in the stellate ganglion, which in turn are in close rela- 

 tion with the fine, medullated, preganglionic fibers coming from the cord, 

 In man these post-ganglionic fibers pass from the inferior cervical 

 ganglion and the annulus in three groups and enter the cardiac plexus, 

 whence they pass to the muscle-cells. 



Stimulation with electricity of these branches of the cervical ganglion 

 causes in the dog an increase in the frequency of the pulse of even 

 75 per cent, of its normal rate, while the force of the contraction is also 

 much increased. . The conductivity of the heart-muscle is raised also, 

 and a negative electric variation set up opposite in direction from that 

 of inhibition. The sympathetic is therefore a truly augmentory nerve, 

 its influence from the medulla oblongata probably not only hastening 

 but increasing the force of the beat. It has no power, however, of starting 

 a heart which has stopped all contraction as seen through a microscope. 



