CHAP, iv.] THE VASCULAR MECHANISM. 



257 



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 

 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. 43 represents in a diagrammatic 

 form the essential parts of the sphygmograph, known as Dudgeon's. 

 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 surface and 

 with the support of the radius below so that adequate pressure 

 can be brought to bear by the lever on the artery, is favourable 

 for making observations. It can of course be applied to other 

 arteries. When applied to the radial artery some such tracing as 

 that shewn in Fig. 44 is obtained. At each heart-beat the lever 



FIG. 4-1. PULSE TRACING FKOM THE BADIAL ARTERY OF MAN. 



The vertical curved line, L, gives the tracing which the recording lever made 

 when the blackened paper was motionless. The curved interrupted lines shew the 

 distance from one another in time of the chief phases of the pulse-wave, viz. 

 x = commencement and A end of expansion of artery, p, predicrotic notch, d, di- 

 crotic notch. C, dicrotic crest. D, post-dicrotic crest. /, the post-dicrotic notch. 

 These are explained in the text later on. 



rises rapidly and then falls more gradually in a line which is more 

 or less uneven. 



140. We have now to study the nature and characters of 

 the pulse in greater detail. 



We may say at once, and indeed have already incidentally 

 seen, that the pulse is essentially due to the action of physical 

 causes; it is the physical result of the sudden injection of the 

 contents of the ventricle into the elastic tubes called arteries ; its 

 more important features may be explained on physical principles 

 and may be illustrated by means of an artificial model. 



If two levers be placed on the arterial tubes of an artificial 

 model Fig. 30 S. a., S'. a., one near to the pump, and the other 



F. 17 



