THE CARDIAC NERVES. 



591 



the beat originates, the theory offers a simple explanation of the stoppage of 

 the beat, of the quick recovery after stimulation ceases, and of the retention 

 of irritability to direct stimula- 

 tion shown by the heart during 

 vagus inhibition. A heart that 

 has been stopped by an excess 

 of potassium chloride added to 

 the circulating liquid beats very 

 promptly as soon as the excess 

 of the potassium is removed, and 

 as in the case of vagus inhibition 

 it seems often to show a notice- 

 able improvement in condition. 



That the inhibitory ef- 

 fect of the vagus im- 

 pulses upon the heart is 

 not due to any peculiarity 

 in properties of these 

 fibers or of the impulses 

 themselves, but is depend- 

 ent rather upon the place 

 or manner of ending in the 

 heart, has been demon- 

 strated by direct experi- 

 ment. Erlanger* has shown 

 that when an ordinary 

 spinal nerve (fifth cervical) 

 is sutured to the peripheral 

 end of the cut vagus, it will, 

 after time for regeneration 

 has been allowed, cause, 

 when stimulated, the usual 

 stoppage of the heart. 



The Course of the Ac- 

 celerator Fibers. The 

 heart receives efferent or 

 motor nerve fibers from 

 the sympathetic system in 

 addition to those reaching it by way of the vagus nerve. Atten- 

 tion was first called to these sympathetic fibers by Legallois 

 (1812), but our recent knowledge dates from the experiments 

 made by von Bezold (1862), which were afterward completed 

 by the Cyon brothers M. and E. Cyon f 1866. These fibers 

 when stimulated cause an increased rate of beat and are, there- 

 fore, designated as the accelerator nerve of the heart. Their 



* Erlanger, u American Journal of Physiology," 13, 372, 1905. 



t For the history and literature of the accelerator nerves, see Cyon, article 

 "Cceur," p. 103, in Richet's " Diction naire de Physiologic," 1900; or Tiger- 

 etedt, "Lehrbuch der Physiologic des Kreislaufes," 260, 1893. 



Fig. 243. Schematic representation of the 

 course of the accelerator fibers to the dog's heart 

 right side. (Modified from Pawl&w.) 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; //, ///, 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. 



