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 when excited they cause a 

 fall of blood-pressure by reflex action upon the vasomotor center. 

 For this reason they are described as depressor nerve fibers. These 

 latter fibers may run as a separate nerve or may be included in the 

 trunk of the vagus. 



The Course of the Cardiac Fibers. The vagus nerve gives 

 off several branches that supply the heart. The superior cardiac 

 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 and indeed some 

 of these branches may spring directly from the inferior laryngeal. 

 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 autonomic fibers 

 (p. 234) through the vagus nerve. The preganglionic fibers doubt- 

 less 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. 212 and 213. 



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