BY DR. BURDON-SANDERSON. 237 



individuals, and under certain conditions of health, the instan- 

 taneously transmitted vibration-effect is more felt by the fin- 

 ger; in others, the moment at which the artery attains its 

 greatest extension. Thus a pulse of the form shown in Fig. 

 215 a seems to the finger delayed, because the vibration-effect 

 is in abeyance on account of the existence of an obstruction 

 between the heart and the wrist; whereas, in the pulse re- 

 presented in 6, the initial shock is so intense that it masks 

 the other. 



43. Cause of the Second Beat. The facts relating to 

 the postponement of arterial expansion are also the key to the 

 understanding of the phenomenon of dicrotism. In applying 

 them in explanation of the production of the second expansion 

 in arteries which, like the radial, are not far from the periphery, 

 there are two facts to be borne in mind: first, that these arte- 

 ries, as they become smaller, become more distensible; and 

 secondly, that in the capillaries themselves the resistance to 

 the passage of blood is much greater than any which is en- 

 countered in the arteries. Just as the expansion of the aorta 

 determines that of the radial, the radial expansion determines 

 and is followed by that of the peripheral arterioles. Hence at 

 a certain moment the radial is subsiding, while the arterioles 

 are still swelling; so that, when they are at their acme of dis- 

 tension, the pressure is greater at the periphery than in the 

 radial itself. From the other fact the resistance to the flow 

 of blood in the capillaries it results that, immediately behind 

 this resistance, pressure accumulates so long as blood enters 

 the arterioles from behind more rapidly than it is discharged 

 in front. The state of the arterial circulation during the period 

 of cardiac diastole may therefore be described as follows: The 

 arterial system is closed by the aortic valve behind, and vir- 

 tually closed in front by the capillary resistance. In the 

 largest arteries the expansion is ebbing, in the smallest it is 

 culminating; so that, fur an instant, the pressure is greater in 

 the latter than in the former. There is but one effect possible. 

 The restoration of equilibrium must take place by increase of 

 pressure towards the heart and diminution towards the peri- 

 phery. This restoration of equilibrium constitutes the second 

 beat. It ma} r manifest itself in veiy different degrees, accord- 

 ing to the yieldingness of the arteries. When, as in health, 

 the arteries are tense, it is seen merely in a slight arrest or in- 

 terruption of the arterial collapse a break in the descending 

 limb of the tracing. In fever, when the arteries are relatively 

 much more distensible, the second expansion is separated by 

 so distinct an interval of relaxation from the first that the pulse 

 feels double to the finger. To facilitate the comprehension of 

 the subject, the synchronous conditions of central, peripheral, 

 and intermediate arteries may be stated in parallel columns. 



