398 



RESPIRATION. 



undulations, the so-called Traube or Traube-Hering curves, make their ap- 

 pearance (Fig. 108, 2, 3), very similar to the previous ones, except that their 

 curves are larger and of a more sweeping character. These new undula- 

 tions, since they appear in the absence of all thoracic or pulmonary move- 

 ments, passive or active, and are witnessed even when both vagi are cut, 

 must be of vasomotor origin ; the rhythmic rise must be due to a rhyth- 

 mic constriction of the small arteries, and this probably is caused by a 

 rhythmic discharge from vasomotor centres, and especially from the medul- 

 lary vasomotor centre. The undulations are maintained as long as the blood- 

 pressure continues to rise. With the increasing venosity of the blood, 

 however, both the vasomotor centres and the heart become enfeebled ; the 

 undulations disappear, and the blood -pressure rapidly sinks. 



FIG. 108. 



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i A A H 



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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 exper- 

 iment. 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 res- 

 piration 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 dis- 

 appeared, and the blood-pressure rose steadily, while the heart-beats became slower. Soon, as 

 shown 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 un- 

 dulations is not dependent on the cessation of the respiratory movements, 

 and on an abnormally venous condition of the blood. They are sometimes 

 (Fig. 109) seen in animals whose breathing is fairly normal. We need 

 not discuss them any further now, and have introduced them chiefly to 

 illustrate the fact that the vasomotor nervous system is apt to fall into a 

 condition of rhythmic activity. It has been suggested that the normal 



