THE CIRCULATION. 191 



study of the pulse of man himself. In Figure 85 there are 

 indicated such sphygmograph curves. 



On animals there is more generally used an instrument 

 called the kymograph. This is in principle a revolving 

 drum run by clockwork on which the recording lever traces 

 the desired curves. This lever rests on the mercury in one 

 limb of a U-shaped tube, while the other limb of this U- 

 shaped tube (D in Figure 86) is directly connected with the 

 cut end of the artery to be examined. To prevent the 

 blood from touching the mercury there is put over it in that 

 part of the tube some liquid which will prevent the blood 

 from clotting. Upon connection with the artery the pres- 

 sure of the blood of course at once forces the mercury down 

 one limb of the tube and necessarily up in the other, but as 

 the recording lever moves up and down with the mercury 

 on which it rests it records all these variations in pressure. 

 An examination of the figure of the kymograph will easily 

 show how a slight increase of arterial pressure will at once 

 cause the recording lever to be raised, while a decrease at 

 once lowers it. On the revolving drum this recording lever 

 will of course then describe curves such as those pictured 

 in Figure 86, which reproduce in excellent detail for per- 

 manent study all the variations of pressure in the artery ex- 

 amined. On such kymograph tracings as those traced on 

 the drum in the figure here given, the small curves indi- 

 cate the individual pulse waves, or what is the same thing 

 finally, the heart beats, while the larger waves indicate the 

 movements of breathing. In the case of a dog the arterial 

 pressure rises through inspiration and sinks during expira- 

 tion. The explanation of this periodic respiratory rise and 

 fall of arterial pressure is not yet possible, nor has it been 

 definitely proved that it exists in man. If we, therefore, 

 since we do not understand fully these respiratory curves, 

 turn our attention to the pulse waves themselves, each is 

 seen to consist of a straight and rapid rise of pressure to a 

 maximum height, while the fall of pressure, or the down- 

 ward wave, is not at all straight and gradual. There is 



