CHAP. IL] RESPIRATION. 653 



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 inspira- 

 tion 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. We cannot as yet explain exactly 

 the manner in which the activity of the one centre influences 

 that of the other; it may be that during the expiratory phase 

 the blood reaching the bulb is not quite so well arterialized, 

 especially so far as the escape of carbonic acid is concerned, as 

 during the inspiratory phase, and that the cardio-inhibitory centre 

 is sufficiently sensitive to appreciate the slight difference ; but of 

 this we cannot be sure. 



386. Many and varied are the changes which take place in 

 the vascular system, bearing upon both the heart and the vaso- 

 motor mechanism, when the blood fails to be duly arterialized. 

 These are shewn in a striking manner and may be profitably 

 studied in an animal in which the interference with respiration 

 is carried as far as asphyxia. The exaggerated respiratory move- 

 ments and convulsive struggles which are characteristic of this 

 condition, introduce mechanical complications such as those which 

 we have just studied ; these however may be readily eliminated 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. 86. The mean 

 pressure 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 more than the double of its pre- 

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

 heart-beats are conspicuous ; these are due on the one hand to the 

 rhythm of the heart being slowed whereby each individual beat 

 produces more effect on the manometer, and on the other hand 

 to the output at each beat, ' the pulse-volume,' as shewn by direct 

 observation with the cardiometer, being increased. The slowing 

 of the rhythm is in part due to vagus inhibitory action, the 

 too venous blood exciting the bulbar cardio-inhibitory centre; 

 for the effect -is much less when both vagus nerves are divided. 

 But as illustrated by Fig. 86, which is a curve of blood-pressure ' 

 during asphyxia after division of both vagus nerves, the effect 

 is not then wholly done away ; the slowing is in part due to other 

 causes, but what these are is not very clear. 



