522 CIRCULATION OF BLOOD AND LYMPH. 



some difference in their place or manner of ending in the muscular 

 tissue? Views differ upon this point and many physiologists have 

 suggested that the impulses vary in quality; that the inhibitory 

 nerve impulse differs in some unknown way from a motor impulse, 

 and therefore causes an opposite reaction in the muscle. This latter 

 view seems, however, to be entirely disproved by the results of 

 experiments. Langley has shown upon blood-vessels (p. 76) that 

 an inhibitory nerve made to grow down a motor path causes when 

 stimulated only motor effects and vice versa. And in the case in point 

 Erlanger* has proved 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. So far as our facts go, therefore, 

 we must assume that motor and inhibitory fibers have opposite 

 effects upon the muscular fibers in which they end because they 

 terminate differently in these fibers. Nothing more specific can be 

 said. 



The Course of the Accelerator 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 therefore designated as the 

 accelerator nerve of the heart. Their course has been worked out 

 physiologically in a number of animals. Among the mammalia and 

 indeed among different animals of the same species there is some 

 variation, but a general conception of their origin and course may 

 be obtained from Fig. 216, which represents in a schematic way the 

 anatomical path taken by these fibers. They emerge from the spinal 

 cord in the anterior roots of the second, third, and fourth thoracic 

 spinal nerves. According to some authors, they may be found also 

 in the fifth thoracic and the first thoracic or even the lower cervical 

 spinal nerves. They pass then by way of the white rami to the 

 stellate or first thoracic ganglion (6), and thence byway of the an- 

 nulus of Vieussens (7) to the inferior cervical ganglion. A num- 

 ber of branches leave the sympathetic system and the vagus in 

 this region to pass to the cardiac plexus and thence to the heart. 

 The accelerator fibers are found in some of these branches, mixed 

 in some cases with inhibitory fibers from the vagus. The pregan- 

 glionic portion of some of the accelerator fibers ends around the 



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



f For the history and literature of the accelerator nerves see Cvon, article 

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

 stedt, " Lehrbuch der Physiologie des Kreislaufes," 260, 1893. 



