CHAP, iv.] THE VASCULAR MECHANISM. 221 



And there are still other instruments which may be used in a 

 similar way. 



It is not necessary, however, to open the artery ; we may study 

 indirectly the changes of pressure by recording the expansions and 

 retractions of the artery, the changes in its diameter, which are 

 produced by the changes of pressure. 



The most common method of registering the expansion of an artery 

 and at the same time one of the simplest, is that of bringing a light lever 

 to bear on the outside of the artery. 



A lever specially adapted to record a pulse tracing is called a 

 sphygmograph, the instrument generally comprising a small travelling 

 recording surface on which the lever writes. There are many different 

 forms of sphygmograph, but the general plan of structure is the same. 

 Fig. 57 represents in a diagrammatic form the essential parts of the 

 sphygmograph known as Dudgeon's, which we have chosen for repre- 

 sentation, not because it is best, but because it is one very largely 

 employed in medical practice. The instrument is generally applied to 

 the radial artery because the arm affords a convenient support to the 

 fulcrum of the lever, and because the position of the artery, near to the 



Fio. 57. DIAGRAM OP A SPHYOMOGRAPH (Dudgeon's). 



Certain supporting parts are omitted so that the multiplying levers may be 

 displayed. 



a is a small metal plate which is kept pressed on the artery by the spring b. 

 The vertical movements of a cause to-and-fn> movements of the lever c about the 

 fixed point d. These are communicated to and magnified by the lever e, which 

 moves round the fixed point f. The free end of this lever carries a light steel 

 marker which rests on a strip of smoked paper q. The paper is placed beneath two 

 small wheels, and rests on a roller which can be rotated by means of clock -work 

 contained in the box h. The paper is thus caused to travel at a uniform rate. 

 The screw graduated in ounces Troy is brought to bear on the spring b bv means <>{ 

 a camm, and by this the pressure put on the artery can be regulated. The levers 

 magnify the pulse movements fifty times. 



