CHAP, ii.) RESPIRATION. 621 



lution and becomes slower during the fall ; see Fig. 85. A similar 

 influence may bo seen in pulse-tracings taken from man. The 

 quickening of the beat might be considered as itself partly 

 accounting for the rise of pressure, or on the other hand it 

 might be urged that the increased flow of blood which causes the 

 rise of pressure, at the same time leads to the quickening of the 

 beat, were it not for one fact, viz. that the difference 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 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 

 indications that, while the respiratory centre in the medulla 

 oblongata 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 medulla is not quite so well arterialized, 

 especially as 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. When through interference with the pulmonary inter- 

 change the blood sent out from the left ventricle becomes and 

 continues to be less arterialized than usual, the effects on both 

 the heart and the vaso-motor system become conspicuous. The 

 rhythm of the heart-beats is most distinctly slowed. This, under 

 ordinary circumstances when the vagus nerves are intact, is 

 probably in part the result of vagus inhibition, the venous blood, 

 as suggested above, stimulating the cardio-inhibitory centre in the 

 medulla. But the slowing is not wholly caused in this way, for 

 it is still conspicuous in an animal placed under urari and with 

 both vagus nerves divided. Compare curves 3 and 4 with 1 and 

 2 in Fig. 86. How this slowing is brought about is not very 

 clear. When venous blood is sent through an excised heart, the 

 beat is, it is true, slowed, but it is also and still more conspicu- 

 ously weakened. Now when the blood becomes too venous, as 

 is shewn in Fig. 86, even after the action of the vagus nerves 

 has been eliminated by section and also by urari, the slowing 

 is out of proportion to the weakening, since, as we shall presently 

 see, the blood-pressure rises ; and though that rise is chiefly 

 due to vaso-motor constriction, still it could not take place if 

 the cardiac stroke were very notably weakened. It may be that 

 the venous blood stimulates the cardiac augmentor mechanism 



