THE PULSE. 



177 



cases, as where an artery has a bend, may cause a certain amount of loco- 

 motion 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. 59), 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 



FIG. 59. 



Fick's Spring Manometer. The flattened tube in the form of a hoop is firmly fixed at one end, 

 while the other free end is attached to a lever. The interior of the tube, filled with spirit, is 

 brought, by means of a tube containing sodium carbonate solution, into connection with an 

 artery, in much the same way as in the case of the mercury manometer. The increase of pressure 

 in the artery being transmitted to the hollow hoop, tends to straighten it, and correspondingly 

 moves the attached lever. 



of pressure or of the expansion of the artery which is the result of that 

 increase. The easiest and most common method of registering the expansion 

 of an artery is that of simply bringing a light lever to bear on the outside 

 of the artery. 



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

 graph, the instrument generally comprising a small travelling recording 

 surface on which the lever writes. There are many different forms of sphyg- 

 mograph, but the general plan of structure is the same. Fig. 60 represents 

 in a diagrammatic form the essential parts of the sphygmograph, known as 

 Dudgeon's [and Fig. 61, Marey's, which is in more common use]. 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 

 12 



