THE CIRCULATION. 281) 



over the knee of the other. The leg in this position represents 

 typically a lever of the third class. 



One observer conceived the idea from this phenomenon that the 

 pulse can be very accurately studied by using a very light lever so 

 attached that it will oscillate at each heart-beat. By virtue of a large 

 arm to the lever the amplitude of the oscillations is so exaggerated 

 that they can be readily seen by the naked eye and their movements 

 graphically depicted upon smoked papers. The instrument capable 

 of determining the various elements of the pulse and so depicting 

 them that they can be studied at leisure has received the name 

 sphygmo graph. 



The Sphygmograph. The name whereby this instrument is 

 known is derived from two Greek words which mean "to write the 



Fig. 92. Marey's Sphygmograph. (EO.) 



The parts B, B, B are fastened to the wrist by the straps B, B. The 

 remaining part of the instrument rests on the forearm. The end of the screw, 

 V, rests on the spring R, the button of which lies on the radial artery. Any 

 movement of the button at R is communicated to V, which moves the lever, 

 L, up and down. When in position the blackened slip of glass is made to move 

 evenly by the clockwork, H, so that it records the movements of the lever. 



pulse/' It does write, for to-day graphic records of the various fea- 

 tures of the pulse are obtained by its use. 



The essential feature of this instrument is its system of com- 

 pound levers whereby the initial motion is multiplied about fifty 

 times. The foot of these levers rests upon the skin over the artery 

 whose tracing is to be taken. Motion is transmitted from it to the 

 other end of the levers, where is inserted a recording needle. 



The second feature of the apparatus is the recording instrument, 

 composed of clock-work, which revolves a pair of small cylinders 

 between which is moved a ribbon of blackened paper. The record- 

 ing-needle's point rests upon this paper, correctly depicting there the 

 various features of the pul?e. 



In addition, each instrument is provided with an apparatus by 

 adjustment of which the pressure is so regulated that the best record 



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