464 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



rate and lower the blood-pressure. 1 Observers differ as to the results of stim- 

 ulation of the central end of the laryngeal branches of the vagus on the pulse- 

 rate and blood-pressure. 2 



Depressor Nerve. The earlier stimulations of the nerves that pass 

 between the central nervous system and the heart, with the exception of the 

 vagus, altered neither the blood-pressure nor the pulse-rate. Ludwig and Cyon 3 

 suspected that the negative results were owing to the fact that the stimulations 

 were confined to the end of the cut nerve in connection with the heart. Some 

 of the nerves, they thought, should carry impulses from the heart to the brain, 

 and such nerves could be found only by stimulation of the brain end of the 

 cut nerve. They began their research for these afferent nerves with the branch 

 which springs from the rabbit's vagus high in the neck and passes downward 

 to the ganglion stellatum. Their suspicion was at once confirmed. The stimu- 

 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. 4 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 



1 Franck, 1880, p. 378. 2 Aubert and Koever, 1868, p. 241 ; Franck, 1880, p. 367. 



3 Ludwig and Cyon, 1866, p. 128. 



4 Tschirwinsky, 1896, p. 778, gives a somewhat different account. 



