270 A MANUAL OF PHYSIOLOGY. 



deep and slow respiration. It is caused by a rhythmical rise and 

 fall in the activity of the cardio-inhibitory centre, synchronous 

 with the respiratory movements, for the difference disappears 

 after division of both vagi. The normal respiratory oscillations 

 of blood-pressure are not due in any great degree to such changes 

 in the rate of the heart, for they persist after section of the vagi, 

 and they are seen in animals like the rabbit, in which in ordinary 

 breathing little or no variation in the rate of the heart is con- 

 nected with the phases of respiration. The most probable ex- 

 planation of the respiratory variations in the pulse-rate is that 

 the respiratory centre, when it is discharging itself in inspiration, 

 sends out impulses as a sort of overflow along fibres connecting 

 it with the cardio-inhibitory centre. These increase the tone of 

 that centre, but, owing to the long latent period of the cardio- 

 inhibitory apparatus, the inhibition does not reveal itself till the 



FIG. 115. BLOOD-PRESSURE TRACING : RABBIT, UNDER CHLORAL. 

 Natural respiration stopped at I in inspiration, at E in expiration. The mean 

 blood -pressure is scarcely altered ; but the respiratory waves become much larger 

 owing to the abortive efforts at breathing. Time tracing shows seconds. 



succeeding expiration. It may be, however, that the impulses 

 discharged from the respiratory centre in inspiration diminish 

 the tone of the cardio-inhibitory centre, and thus lead to accelera- 

 tion of the heart towards the end of the inspiratory phase. 

 In certain pathological conditions the influence of the respira- 

 tion on the pulse-rate is exaggerated (so-called respiratory 

 arhythmia) . 



Traube-Hering Curves. Rhythmical changes in the activity 

 of the vaso-motor centre, also associated with periodic discharges 

 from the respiratory centre, may be observed under certain con- 

 ditions e.g., when in an animal paralyzed by curara, and 

 therefore unable to breathe spontaneously, the artificial respira- 

 tion is stopped for a time. If such a dose of curara be given as 

 will still permit slight spontaneous respiration to go on, and 

 both vagi be cut, it can be seen on stopping the artificial respira- 

 tion that the waves on the blood-pressure curve are exactly 

 synchronous with the slow respiratory movements. The 



