6o 



THE MECHANISM OF THE CIRCULATION. 



although it has been diligently sought for by several observers. In man, 

 the homologous nerve arises from the junction of the vagus and superior 

 laryngeal nerves, but quickly joins again the main trunk of the vagus. 



The depressor seems to exert no tonic action on the vasomotor 

 centre. Neither during normal conditions, nor in the state of asphyxia, 

 is any rise of arterial pressure produced by sudden section of both 

 depressors. 1 The depressor seems incapable of fatigue. It has main- 

 tained its full functional effect during a period of excitation lasting 

 seventeen minutes. 2 



If the vagi be cut, excitation of the depressor is often followed by 

 acceleration of the heart. This is the ordinary effect of a fall of blood 

 pressure. If, while one depressor is under excitation, the other be 

 stimulated, a summation of effect is produced. 



The activity of the depressor abolishes or very much diminishes 

 Traube-Hering curves, when these are present on the tracing of arterial 

 pressure. At the same time, it lessens the extent of the respiratory 

 undulations. On simultaneously exciting a pressor nerve, such as the 

 anterior crural, together with the depressor, an antagonism is provoked, 



Fig. 40. — Aortic pressure. Excitation of depressor. The drum was 

 stopped in the middle of the curve, and the excitation maintained 

 for seventeen minutes. The line of zero pressure (0. 0) should be 

 30 mm. lower than is here shown. — Bayliss. 



and the strongest produces its effect. So when the depressor is excited, 

 and asphyxia produced, the pressor effect overcomes and abolishes the 

 depressor influence, while, if the excitation be continued, and respiration 

 be supplied, the depressor effect, after a short period, regains the 

 mastery. 3 Neither pressor nor depressor effects can be obtained after 

 the injection of strychnine, nor does the depressor act while the arteries 

 are contracted by intravenous injection of suprarenal extract. 4 



Excitation of the depressor increases the resistance in the central 

 nervous system in such a way as to prevent the tonic influence which 

 is maintained by the vaso-constrictor centre. It does not appear, how- 

 ever, to produce anabolism of this centre during the period of rest or 

 inhibition, for the rise of arterial tension produced by asphyxia is no 

 greater after than before stimulation of the depressor. 



1 Konow and Stenbeck, Skandin. Arch. f. Physiol., Leipzig, 1889, Bd. i. S. 424. 



2 Bayliss, Journ. Physiol., Cambridge and London, 1893, vol. xiv. p. 314. 



3 Ibid., p. 319. 



4 Oliver and Sehafer, Journ. Physiol., Cambridge and London, 1895, vol. xviii. 



