CIRCULA TION. 1 73 



lation of the central end of this nerve, called by Ludwig and Cyon the depres- 

 sor, caused a considerable fall of the blood-pressure. 



The depressor nerve arises in the rabbit by two roots, one of which comes 

 from the trunk of the vagus itself, the other from a branch of the vagus, the 

 superior laryngeal nerve. Frequently the origin is single ; in that case it is 

 usually from the nervus laryngeus. 1 The nervus depressor runs in company 

 with the sympathetic nerve to the chest, where communications are made with 

 the branches of the ganglion stellatum. 



The stimulation of the peripheral end of the depressor nerve is without 

 effect on the blood-pressure and heart-beat. The stimulation of the central 

 end, on the contrary, causes a gradual fall of the general blood-pressure to the 

 half or the third of its former height. After the stimulation is stopped, the 

 blood-pressure returns gradually to its previous level. 



Simultaneously with the fall in blood-pressure a lessening of the pulse-rate 

 sets in. The slowing is most marked at the beginning of stimulation, and after 

 rapidly reaching its maximum gives way gradually until the rate is almost 

 what it was before the stimulation began. After stimulation the frequency is 

 commonly greater than previous to stimulation. 



After section of both vagi, the stimulation of the depressor causes no change 

 in the pulse-rate, but the blood-pressure falls as usual. The alteration in fre- 

 quency is therefore brought about through stimulation of the cardiac inhibitory 

 centre, acting on the heart through the vagi. The experiment teaches, further, 

 that the alteration in pressure is not dependent on the integrity of the vagi. 



Poisoning with curare paralyzes all motor mechanisms except the heart and 

 the muscles of the blood-vessels. Yet curare-poisoning does not affect the 

 result of depressor stimulation. The cause of the fall in blood-pressure must 

 be sought then either in the heart or the reflex dilatation of the blood-vessels. 

 It cannot be in the heart, for depressor stimulation lowers the blood-pressure 

 after all the nerves going to the heart have been severed. It must therefore 

 lie in the blood-vessels. Ludwig and Cyon knew that the dilatation of 

 the intestinal vessels could produce a great fall in the blood-pressure and 

 turned at once to them. Section of the splanchnic nerve caused a dilata- 

 tion of the abdominal vessels and a fall in the blood-pressure. Stimula- 

 tion of the peripheral end of the cut splanchnic caused the blood-pressure to 

 rise even beyond its former height. Ludwig and Cyon reasoned that if the 

 depressor lowers the blood-pressure el. icily by affecting the splanchnic uerve 

 lvllexlv, the stimulation of the central end of the depressor after section of 

 the splanchnic nerves ought to have little effeel on the blood-pressure. This 

 proved to be the case. The investigators concluded thai the depressor re- 

 duces the blood-pressure chiefly by lessening the tonus of the vessels governed 

 by the splanchnic nerve, thus allowing their dilatation and in consequence 

 lessening the peripheral resistance. The fallacy in this argument has re- 

 cently been pointed out l>v Porter and Beyer. 2 The stimulation of the >\<'- 



1 Tseliirwinsky : Centralblatt fur Physiologie, 1896, i\. \< 778, gives :> somewhat different 

 account. 



* Porter and Beyer: American Journal of Physiology, L900, \\iii. 



