RESPIRATION. 291 



during expiration, except at its beginning, this property of theirs acts in 

 the opposite direction, and diminishes the flow. Thus, at the beginning 

 of inspiration the diminution of blood-pressure, which commenced during 

 expiration, is continued, but after a time the diminution is succeeded by 

 a steady rise; the reverse is the case with expiration at first a rise and 

 then a fall. 



The effect of the nervous system in producing rhythmical altera- 

 tions quite independent of the mechanically caused undulations of the 



Fig. 218. Traube-Hering's curves. (To be read from left to right.) The curves 1, 2, 3, 4, and 5 

 are portions selected from one continuous tracing forming the record of a prolonged observation, 

 so that the several curves represent successive stages of the same experiment. Each curve is placed 

 in its proper position relative to the base line, which is omitted ; the blood-pressure rises in stages 

 from 1 to 2, 3, and 4, but falls again in stage 5. Curve 1 is taken from a period when artificial res- 

 piration was being kept up, but the vagi having been divided, the pulsations on the ascent and de- 

 scent of the undulations do not differ; when artificial respiration ceased these undulations for a 

 while disappeared, and the blood-pressure rose steadily while the heart-beats became slower. Soon, 

 as at 2, new undulations appeared ; a little later, the blood-pressure was still rising, the heart beats 

 still slower, but the undulations still more obvious (3); still later (4), the pressure was still higher, 

 but the heart-beats were quicker, and the undulations flatter, the pressure then began to fall rapidly 

 (5), and continued to fall unal some time after artificial respiration was resumed. (M. Foster.) 



blood-pressure is two-fold. In the first place the cardio-inhibitory centre 

 is stimulated during the fall of blood-pressure, and produces a slower 

 rate of heart-beat, which will be noticed in the tracing (fig. 218). The 

 undulations during the decline of blood-pressure are therefore longer 

 but less frequent. This effect disappears when, by section of the vagi, 

 the effect of the centre is cut off from the heart. In the second place, 

 the vaso-molor centre sends out rhythmical impulses, by which undula- 

 tions of blood-pressure are produced, quite independent of the so-called 



