452 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



In the rabbit the cervical sympathetic and the vagus trunk are not joined, 

 as in the dog, but run a separate course. Cardiac fibres from the spinal cord 

 reach the lower cervical and first thoracic ganglion (ganglion stellatum) along 

 their rami communicantes l and pass to the heart by two sympathetic cardiac 

 nerves, one from the inferior cervical ganglion and one from the ganglion 

 stellatum. 2 



The arrangement of the cardiac nerves in the cat is shown in Figure 114. 



In the frog the cardiac nerves, both vagal and sympathetic, reach the heart 

 through the splanchnic branch of the vagus. The sympathetic fibres pass out 

 of the spinal cord with the third spinal nerve, through the ramus communicaus 

 of this nerve into the third sympathetic ganglion, 3 up the sympathetic chain 

 to the ganglion of the vagus, and down the vagus trunk to the heart. 4 



THE INHIBITORY NERVES. 



In 1845, Ernst Heinrich and Eduard Weber 5 announced that stimulation 

 of the vagus nerves or the parts of the brain where they arise slows the heart 

 even to arrest. When one pole of an induction apparatus was placed in the 

 nasal cavity of a frog and the other on the spinal cord at the fourth or fifth 

 vertebra, the heart was completely arrested after one or two pulsations and 

 remained motionless several seconds after the interruption of the current. 

 During the arrest, the heart was relaxed and filled gradually with blood. 

 When the stimulus was continued many seconds, the heart began to beat again, 

 at first weakly and with long intervals, then more strongly and frequently, 

 until at length the beats were as vigorous and as frequent as before, though all 

 this time the stimulation was uninterrupted. 



In order to determine from what part of the brain this influence proceeds, 

 the electrodes were brought very near together and placed upon the cerebral 

 hemispheres. The movements of the heart were not affected. Negative results 

 followed also the stimulation of the spinal cord. Not until the medulla oblon- 

 gata between the corpora quadrigernina and the lower end of the calamus scrip- 

 torius was stimulated did the arrest take place. Cutting away the spinal cord 

 and the remainder of the brain did not alter the result. 



Having determined that the inhibitory power had its seat in the medulla 

 oblongata, the question arose through what nerve the inhibitory influence is 

 transmitted to the heart. In a frog in which the stimulation of the medulla 

 had stopped the heart, the vagus nerves were cut and the ends in connection 

 with the heart stimulated. The heart was arrested as before. 



Thus the fundamental fact of the inhibition of a peripheral motor mechan- 

 ism by the central nervous system through the agency of special inhibitory 



1 Bever and von Bezold, 1867, pp. 236, 247. 



2 Ludwig and Thiry, 1864, p. 429 ; Bever, 1867, p. 249. 



3 It is probable that the fibres of spinal origin end in the sympathetic ganglia, making con- 

 tacts there with sympathetic ganglion-cells, the axis-cylinder processes of which pass up the 

 cervical chain and descend to the heart in company with the vagus. 



4 Gaskell and Gadow, 1884, p. 369. 5 E. Weber, 1846, p. 42. 



