CHAPTER XXX. 



THE CARDIAC NERVES AND THEIR PHYSIOLOGICAL 



ACTION. 



The heart receives two sets of efferent nerve fibers from the 

 central nervous system. One set reaches the heart through the 

 vagus nerves, and, since their activity slows or stops the heart 

 beat, they are spoken of as the inhibitory nerve fibers. The other 

 set passes to the heart by way of the sympathetic chain, and since 

 their activity accelerates or augments the heart beat they are 

 designated usually as the accelerator nerve fibers. In addition the 

 heart is provided with a set of afferent nerve fibers. Regarding 

 the functional activity of these latter fibers, our experimental 

 knowledge is limited to the fact that some of them, at least, 

 are stimulated at each beat of the heart (p. 614), and -that 

 possibly some of them help to form the so-called depressor 

 nerve (p. 614). Under pathological conditions these afferent 

 fibers may produce painful sensations. 



The Course of the Cardiac Fibers. The vagus nerve gives 

 off several branches that supply the heart. The superior car- 

 diac branches arise from the vagus in the neck somewhere 

 between the origins of the superior and the inferior laryngeal 

 nerves. The inferior cardiac branches arise from the thoracic 

 portion of the vagus near the origin of the inferior laryngeal 

 (N. recurrens) and, indeed, some of these branches may spring 

 directly from the latter nerve. The inhibitory fibers probably 

 arise in these inferior branches chiefly. Both superior and 

 inferior cardiac branches pass toward the heart and unite 

 with the cardiac branches from the sympathetic chain to form 

 the cardiac plexus. This plexus lies on the arch and 

 ascending portion of the aorta, and from it the heart receives 

 directly both its inhibitory and accelerator fibers. The inhibitory 

 fibers of the heart form a part of the outflow of bulbar autonomic 

 fibers (p. 255) through the vagus nerve. The preganglionic fibers 

 probably end around ganglion cells in the heart, which in turn send 

 their axons as postganglionic fibers to the heart muscle. 



The Action of the Inhibitory Fibers. If the vagus nerve 

 in the neck of an animal is cut and its peripheral end is stimulated 

 the heart is slowed or stopped altogether according to the strength 

 of the stimulus. This effect is illustrated in Figs. 239 and 240. 



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