THE VASCULAR MECHANISM. 



205 



FIG. 73. Dudgeon's sphygmograph, 

 slightly diagrammatic. Explanatior 

 of figures in text. 



each beat is forced through the arterioles during the beat, and much of 

 the force of the heart is expended in further expanding the already 

 distended arterial system to accommodate the extra blood sent into it. 

 The expansion of the arteries starts at the root of the aorta, and 

 proceeds as a wave along the whole arterial system, gradually dying 

 away before the capillaries are 

 reached. This wave of expansion 

 constitutes the pulse. It travels at 

 a rate of 6 to 8 metres a second, 

 and is independent of the movement 

 of the main mass of blood along the 

 arteries, the velocity of which rarely 

 exceeds half a metre a second. 



The pulse can be felt, and often 

 seen, in the superficial arteries of 

 the body, e.g. the radial artery ; in 

 order to study it more exactly a 

 graphic record may be obtained by 

 means of an instrument known as 

 a sphygmograph, of which many 

 forms exist. 



Dudgeon's sphygmograph, which is illustrated diagrammatically in 

 fig. 73, may be attached by a band round the wrist in such a way that 

 the small metal plate A rests on the skin over the radial artery. The 

 movements of the arterial wall are magnified by the series of levers, 



a, b, c, and are recorded by the 

 free end of the lever c, which 

 writes on a moving strip B of 

 blackened (smoked) paper. The 

 paper is moved by means of a 

 small clockwork arrangement; 

 and by means of the dial D 

 the pressure of the plate A on 

 the artery can be adjusted. 



A typical pulse tracing thus 



obtained is seen in fig. 74. It shows a sharp rise from a to b, 

 succeeded as a rule by a steady fall, interrupted at c by a small 

 notch which is immediatel} 7 followed by a slight wave. The rise a to 

 b constitutes the primary or percussion wave ; the wave following 

 c is the dicrotic wave, the notch just preceding it being the dicrotic 

 notch. Other small waves, some preceding the dicrotic wave (pre- 

 dicrotic), and others following it (post-dicrotic), sometimes occur ; they 



FIG. 74. Pulse tracing from the 

 radial artery. 



