VASODILATOR NERVES 217 



Hering curve. They continue until the vaso-motor center is asphyxiated 

 and the heart exhausted, when the pressure falls. The undulations cannot 

 depend upon anything but the vaso-motor center, as the mechanical effects 

 of respiration have been eliminated by the curari and by the cessation of 

 artificial respiration, and the effect of the cardio-inhibitory center has been 

 removed by the division of the vagi. The rhythmic rise of blood pressure 

 is most likely due to a rhythmic constriction of the arterioles followed by a 



FIG. 206. Traube- Bering's Curves. (To be read from left to right.) The curves i, 2, 3, 4. 

 and 5 are portions selected from one continuous tracing forming the record of a prolonged observa- 

 tion, so that the several curves represent successive stages of the same experiment. Each curve 

 is placed in its proper position relative to the base line, which is omitted; the blood pressure rises 

 in stages from i to 2, 3, and 4, but falls again in stage 5. Curve i is taken from a period when arti- 

 ficial respiration was being kept up, but, the vagi having been divided, the pulsations on the ascent 

 and descent of the undulations do not differ; when artificial respiration ceased, these undulations 

 for a while disappeared, and the blood pressure rose steadily while the heart-beats became slower. 

 Soon, as at 2, new undulations appeared; a little later, the blood pressure was still rising, the heart- 

 beats still slower, but the undulations still more obvious (3); still later (4), the pressure was still 

 higher, but the heart-beats were quicker, and the undulations flatter; the pressure then began to fall 

 rapidly (5), and continued to fall until some time after artificial respiration was resumed. (M. 

 Foster.) 



rhythmic fall of pressure and relaxation, both being due to the action of the 

 vaso-motor center. The vaso-motor center, therefore, is capable of pro- 

 ducing rhythmical undulations of blood pressure. 



Vaso-dilator Nerves. Claude Bernard discovered that the blood 

 flow was increased through the salivary glands by stimulation of the nerves 

 (the chorda tympani for the submaxillary, and the tympanic branch of the 

 glossopharyngeal for the parotid), thus proving that the arteries have not 



