KESPIKATION. 



203 



materially aid the circulation, yet under ordinary conditions neither does 

 it obstruct it. Under extraordinary conditions, however, as in violent 

 expirations, the circulation is decidedly obstructed. But we have seen 

 that there is no exact correspondence between the points of extreme 

 arterial tension and the end of inspiration, and we must look to the ner- 

 vous system for an explanation of this apparently contradictory result. 



The effect of the nervous system in producing a rhythmical alteration 

 of the blood-pressure is twofold. In the first place the car die-inhibitory 

 centre is believed to be stimulated during the fall of blood-pressure, pro- 

 ducing a slower rate of heart-beats during expiration, which will be 



FIG. 163. Traube-Hering's curves. (To be read from left to right.) The curves 1, 2, 3, 4, and 5 

 are portions selected from one continuous tracing forming the record 9f a prolonged observation, 

 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 1, to 2, 3, and 4, but falls again in stage 5. Curve 1 is taken from a period when artificial res- 

 piration was being kept up, but the vagi having been divided, the pulsations on the ascent and de- 

 scent of the undulations do not differ; when artificial respiration ceased these undulations fora 

 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. Fos- 



noticed in the tracing (Fig. 162). The undulations during the decline 

 of blood-pressure being longer but less frequent, this effect disappears 

 when, by section of the vagi, the effect of the centre is cut off from the 

 heart; and in the second place, the vaso-motor centre is also believed to 



