256 



SPHYGMOGRAPH. 



[BOOK i. 



beat of the heart, is felt, and when a light lever is placed on the 

 artery, the lever is raised at each beat, falling between. 



This intermittent expansion which we call the pulse, cor- 

 responding to the jerking outflow of blood from a severed 

 artery, is present in the arteries only, being, except under 

 particular circumstances, absent from the veins and capillaries. 

 The expansion is frequently visible to the eye, and in some cases, 

 as where an. artery has a bend, may cause a certain amount of 

 locomotion of the vessel. 



The temporary increase of pressure which is the cause of the 

 temporary increase of expansion makes itself felt, as we have seen, 

 in the curve of arterial pressure taken by the mercury manometer; 

 but the inertia of the mercury prevents the special characters of 

 each increase becoming visible. In Fick's spring manometer 

 (Fig. 42), in which the increase of pressure unfolds a curved spring 

 and so moves a lever, the inertia is much less, and satisfactory 

 tracings may be taken by this instrument. Other instruments 

 have also been devised for recording the special characters of each 

 increase of pressure or of the expansion of the artery which is the 

 result of that increase. The easiest and most common method 



b d a' 



FIG. 43. DIAGBAM OF A SPHYGMOGEAPH (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 fro 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 /. The free end of this lever carries a light steel 

 marker which rests on a strip of smoked paper g. 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 by means of 

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

 magnify the pulse movements fifty times. 



