RESPIRATION 249 



bered that in the intact organism the conditions are different 

 (p. 225). 



The Influence of Respiration on the Blood-pressure. We have 

 already stated, in treating of arterial blood-pressure (p. 103), 

 that a normal tracing shows a series of waves corresponding 

 with the respiratory movements. 



When the respiratory movements are recorded simul- 

 taneously with, and immediately below the pressure curve, it 

 is seen that although the mean blood-pressure is falling for a 

 short time at the beginning of inspiration, it soon reaches 

 its minimum, then begins to rise, and continues rising during 

 the rest of this period. At the commencement of expiration 

 it is still mounting, but soon reaches its maximum, begins 

 to fall, and continues falling 

 through the remainder of 

 the expiratory phase. 



The explanations given 

 of this phenomenon are 

 many, but they may all 

 be grouped into two divi- 

 sions, in which nervous and FIG. 86. 



inflnpnrpQ a rp The upper tracing shows the respiratory 

 ' movements in a rabbit ; the lower tracing is 

 invoked as the the blood-pressure curve; I, inspiration; E, 

 expiration, including the pause. 



chief cause. 



Theory of Nervous Influences. Everybody admits that in 

 certain animals (the dog, for instance), and very often, if not 

 constantly, in man, the rate of the heart is greater during 

 inspiration, especially towards its end, than in expiration. 

 This is due to nervous influence, to a rhythmical rise and 

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

 with the respiratory movements, for the difference disappears 

 after division of both vagi. Now, it might be said that the 

 rise of blood-pressure during the latter part of inspiration is 

 simply caused by the increased rate of the heart, which, as 

 we know, can raise the blood-pressure. Nevertheless, this is 

 not the explanation, for the respiratory oscillations persist 

 after section of the vagi, and they are seen in animals like 

 the rabbit, in which little or no variation in the rate of the 

 heart is connected with the phases of respiration. 



