CHAP, iv.] THE VASCULAR MECHANISM. 223 



artery would give some such tracing as that shewn in Fig. 58 which 

 is obtained by means of the sphygrnograph. At each heart beat the 



FIG. 58. PULSE TRACING FROM THE RADIAL 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, f t the post-dicrotic notch. 

 These terms are explained in the text later on. 



curve rises rapidly, and then falls more gradually in a line which 

 is more or less uneven. 



122. 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 physical causes ; it is 

 the physical result of the sudden injection of the contents of the 

 ventricle into the elastic tubes called arteries. Its features 

 depend on the one hand on the systole of the ventricle, on the 

 quantity of blood which is thereby discharged into the aorta, and 

 on the manner in which it is discharged, and on the other hand 

 on the elasticity of the arterial walls. The more important of 

 these features may be explained on physical principles, and may 

 be illustrated by means of an artificial model, so far at least as 

 we can imitate the action of the heart. 



We may confine ourselves, in the first instance, to the simple 

 expansion of the arterial tube and its return to its previous 

 condition, neglecting for the present all secondary events. 



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 

 near to the peripheral resistance, with a considerable length of 

 tubing between them, and both levers be made to write on a 

 recording surface, one immediately below the other, so that their 

 curves can be more easily compared, the following facts may be 

 observed, when the pump is set to work regularly. They are 



