CHAP, iv.] THE VASCULAR MECHANISM. 



273 



artery would give some such tracing as that shewn in Fig. 59 which 

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



FIG. 59. PULSE TBACING FROM THE KADIAL ARTERY OF MAN. 



The vertical curved Ijne, 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 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. 



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 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. 31, 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 



F. 18 



