CHAP. IT.] 



RESPIRATION. 



373 



increasing venosity of the blood, however, both the vaso-motor 

 centres and the heart become exhausted ; the undulations disappear, 

 and the blood-pressure rapidly sinks. 



< -' i / 1 / \ / '. ; I/ \ < I II 1 1 \ I i 



Vl.VV *.P'Ml 



FIG. 61. TBAUBE-HEBING CTTBVES. To be read from left to right. 



The curves 1, 2, 3, 4, 5 are portions selected from one long continuous tracing 

 forming the record of 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, to save space, is omitted ; and it is obvious that, 

 starting from the stage represented by 1, the blood-pressure rises in stages 

 2, 3, and 4, but falls again in stage 5. Curve 1 is taken from a period 

 when artificial respiration was being kept up, and the undulations visible 

 are those the nature of which have been discussed; the vagi having been 

 cut the pulsations on the ascent and descent of the undulations do not differ. 

 When the artificial respiration was suspended these undulations for a while 

 disappeared, and the blood-pressure rose steadily while the heart-beats became 

 slower. Soon, as shewn in curve 2, new undulations appeared. A little later, the blood- 

 pressure was still rising, the heart-beats still slower, but the undulations still more 

 obvious (curve 3). Still later (curve 4), the pressure was still higher, but the heart- 

 beats were quicker, and the undulations flatter. The pressuie then began to fall 

 rapidly (curve 5), and continued to fall until some time after artificial respiration 

 was resumed. 



The appearance of these Traube-Hering curves is not however 

 dependent on the cessation of the respiratory movements, and on 

 an abnormally venous condition of the blood. They sometimes 

 (Fig. 62) are seen in an animal whose breathing is fairly normal. 

 We need not discuss them any further now, and have introduced 

 them chiefly to illustrate the fact that the vaso-motor nervous 

 system is apt to fall into a condition of rhythmic activity. It has 



