372 EFFECTS ON CIRCULATION. [BOOK n. 



change in the undulations, by section of both vagi. Evidently the 

 slower pulse during the fall is caused by a coincident stimulation of 

 the cardio-inhibitory centre in the medulla oblongata, the quicker 

 pulse during the rise being due to the fact that, during that 

 interval, the centre is comparatively at rest. We have here most 

 important indications that, while the respiratory centre in the 

 medulla oblongata is at work, sending out rhythmic impulses of 

 inspiration and expiration, the neighbouring cardio-inhibitory 

 centre is, as it were by sympathy, thrown into an activity of such a 

 kind that its influence over the heart waxes and wanes with each 

 respiratory movement. 



But if the cardio-inhibitory centre is thus synchronously 

 affected, ought we not to expect that the vaso-motor centre should 

 also be influenced ? We have indeed evidence that the action of 

 the vaso-motor centre is largely dependent on the respiratory 

 changes of blood. 



When artificial respiration is stopped, a very large but steady 

 rise of pressure is observed. This may be in part due to the 

 increased force of the cardiac beat, caused by the increasingly 

 venous character of the blood ; but only in part, and that a small 

 part. The rise so witnessed is very similar to that brought about 

 by powerfully stimulating a number of vaso-constrictor nerves ; and 

 there can be no doubt that it is due to the venous blood stimu- 

 lating the vaso-motor centre in the medulla, and thus causing 

 constriction of the small arteries of the body, particularly perhaps 

 those of the splanchnic area. We say ' stimulating the medullary 

 vaso-motor centre,' because, though we must admit that, since a rise 

 of pressure follows upon dyspnoea when the spinal cord has been 

 previously divided below the medulla, the venous blood may 

 stimulate other vaso-motor centres in the spinal cord and possibly 

 even act directly on local peripheral mechanisms, or on the 

 muscular coats of the small arteries themselves, yet the fact 

 that the rise of pressure is much less under these circum- 

 stances shews that the medullary centre plays the chief part. 

 Upon the cessation of the artificial respiration, the respiratory 

 undulations cease also, so that the blood -pressure curve rises 

 at first steadily in almost a straight line ; yet after a while new 

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

 their appearance (Fig. 61. 2, 3), very similar to the previous ones, 

 except that their curves are larger and of a more sweeping 

 character. These new undulations, since they appear in the 

 absence of all thoracic or pulmonary movements, passive or active, 

 and are witnessed even when both vagi are cut, must be of vaso- 

 motorial origin; the rhythmic rise must be due to a rhythmic 

 constriction of the small arteries, and this probably is caused 

 by a rhythmic discharge from vaso-motor centres and especially 

 from the medullary vaso-motor centre. The undulations are main- 

 tained as long as the blood -pressure continues to rise. With the 



