THE DEPRESSOR NERVE. 



59 



the concomitant rise of arterial tension, so that after a time inhibition 

 takes the place of acceleration. In some cases the vagal effect may 

 be the stronger from the first. 1 The frequency of the heart can 

 similarly be affected by stimulation of every other afferent nerve in 

 the body. The nasal branch of the trigeminus seems to stand alone 

 as the one afferent nerve which always produces slowing and never 

 acceleration of the pulse. 



THE DEPRESSOR NERVE. 



In the rabbit, from the junction of the vagus and superior laryngeal 

 nerves, there arises by two roots a fine nerve, which courses down the neck. 



On excitation of the central end of this nerve, Ludwig and Cyon 2 

 witnessed a surprising fall of arterial pressure, accompanied by a slight 

 decrease in cardiac frequency. The peripheral end failed to give any 

 response to stimulation. Thus the purely afferent nature of the nerve 

 was established. On repeating the excitation of the central end, after 

 section of both vagi, the arterial pressure 

 fell to an undiminished extent, but the 

 pulse rate, in this case, remained un- 

 changed. After section of all the cardiac 

 nerves, the result was still the same. This 

 nerve was named, by Ludwig and Cyon, 

 the depressor, for it possessed the pow r er 

 of depressing the arterial tension by 30 

 to 50 per cent. Ludwig and Cyon observed 

 that, on stimulating the depressor, the 

 kidney flushed red with blood, while the 

 fall of arterial tension became insigni- 

 ficant after section of the splanchnic 

 nerves. 



In the lower parts of its course the 

 depressor nerve joins the nervous strands 

 which issue from the first thoracic or 

 stellate ganglion ; thence, passing behind 

 the aorta, and through the connective 

 tissue which lies between the latter and 

 the pulmonary artery, it reaches the heart. 

 ~"ie fibres eventually gain the cardiac 



, and course over the ventricle. At 

 this point they have been stimulated by 

 Wooldridge. 3 Some depressor fibres are 

 found to run in the vagi of the rabbit. 



In different mammals, many variations are to be marked in the 

 course of this nerve, but, in all, fibres with a depressor function are 

 to be found. The same arrangement as in the rabbit, is met with in 

 horses, hares, pigs, and hedgehogs. In dogs, and sometimes in cats, the 

 depressor fibres are bound up with the vago-sympathetic trunk, but can 

 often be separated as a distinct strand from the rest of that nerve. No 

 sign of a depressor nerve has been found in any cold-blooded animal, 



1 Roy and Adanii, Phil. Trans., London, 1892, vol. clxxxiii. B, p. 258. 



2 Eer. d. k. sacks. Gesellsch. d. Wissensch. math.-phys. Cl. t Leipzig, 1866, S. 307. 



3 Arch. f. PhysioL, Leipzig, 1883, S. 539. 



FIG. 39. Dissection of the depressor 

 nerve, rabbit. 1, sympathetic 

 (the white line next to it is 

 the line of the carotid artery) ; 

 2, hypoglossal ; 3, descending 

 branch of hypoglossal ; 4, branch 

 to this from cervical plexus ; 

 5, vagus ; 6, origin of superior 

 larnygeal ; 7 and 8, origin of de- 

 pressor from vagus and superior 

 laryngeal. Ludwig and Cyon. 



