CHAP, ii.] 



RESPIRATION. 



623 



undulations are maintained as long as the blood-pressure con- 

 tinues to rise. With the increasing venosity of the blood, 

 however, both the vaso-motor centres and the heart become 

 enfeebled; the undulations disappear, and the blood-pressure 

 rapidly sinks. 



Fm. 86. 



BLOOD-PRESSURE CURVES DURING A SUSPENSION OF BREATHING. 

 TRAUBE-HERING CURVES. 



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 vagus nerves having been cut the 

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

 the artificial respiration was suspended these undulations 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 pressure then began to fall rapidly (curve 5), 

 and continued to fall until some time later artificial respiration was resumed. 



We may here incidentally remark that the occurrence of 

 long slow undulations is not dependent on the cessation of the 

 respiratory movements, and on an abnormally venous condition 

 of the blood. They are sometimes (Fig. 87) 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 



