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HAND-BOOK OF PHYSIOLOGY. 



ducing a slower rate of heart-beats during expiration, which will be 

 noticed in the tracing (Fig. 163), the undulations during the decline of 

 blood-pressure being 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-motor centre is also believed to send 

 out rhythmical impulses, by which undulations of blood pressure are pro- 

 duced independently of the mechanical effects of respiration. 



FIG. 164. 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 respiration 

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

 dulations 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 until some time after artificial respiration was resumed. (M. Foster.) 



The action of the vaso-motor centre in taking part in producing 

 rhythmical changes of blood-pressure which are called respiratory, is 

 shown in the following way: In an animal under the influence of urari, 

 record of whose blood-pressure is being taken, and where artificial respi- 

 ration has been stopped, and both vagi cut, the blood-pressure curve rises 

 at first almost in a straight line; but after a time new rhythmical undula- 

 tions occur very like the original respiratory undulatious, only somewhat 



