504 RESPIRATORY UNDULATIONS. [Book ii. 



in natural and in artificial respiration, the features of the blood- 

 pressure curve vary according as the breathing is hurried or 

 slow, shallow or deep, and according to the facility with which 

 air enters the chest. So much so indeed is this the case that 

 at times the blood-pressure curves of natural and artificial 

 respiration may closely resemble each other. 



§ 314. We have even in normal quiet breathing indications 

 of another kind of influence of respiration on the beat of the 

 heart. One striking feature of the respiratory undulation in 

 the blood-pressure curve of the dog is the fact that the pulse- 

 rate is quickened during the rise of the undulation and becomes 

 slower during the fall ; see Fig. 99. A similar influence may be 

 seen in the blood-pressure curves of some other animals, but is 

 slight or absent in others, such as the rabbit ; it may be recog- 

 nized in pulse-tracings taken from man. Now this influence is 

 at once done away with, without any other essential change in the 

 undulations, by section of both vagus nerves. Evidently the 

 slower pulse during the fall is caused by a coincident stimulation 

 of the cardio-inhibitory centre in the spinal bulb, the quicker pulse 

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

 the centre is comparatively at rest. We have here indications 

 that, while the respiratory centre in the spinal bulb 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 with each expiration and wanes with each inspiration. 



§ 315. Besides the mechanical effects of the respiratory 

 movements the vascular system is influenced by respiration 

 through the changes in the gases of the blood. The many 

 and varied changes which take place in the vascular system 

 when the blood fails to be duly arterialized are well 

 brought out by a study of asphyxia. The exaggerated 

 respiratory movements and convulsive struggles which are 

 characteristic of this condition, introduce mechanical and 

 other complications which it may be well in the first instance 

 to eliminate ; this can readily be done by placing the animal 

 under urari. If in an animal (dog) under urari the artificial 

 respiration, necessary under the circumstances for the due 

 arterialization of the blood, be stopped the blood-pressure 

 curve soon shews striking changes, cf. Fig. 100. The mean pres- 

 sure after a brief period, the length of which depends on the 

 character of the previous artificial respiration, begins to rise, 

 and continues to rise, at first slowly, afterwards more rapidly, 

 until finally it may reach the double or even more of its pre- 

 vious height. On the curve of pressure the indications of the 

 heart-beats are conspicuous. This is due on the one hand to 

 the rhythm of the heart being slowed, and on the other hand 

 to the output at each beat, as shewn by direct observation 



