MECHANICS OF THE CIRCULATION IN THE VESSELS 107 



narrowed from disease of the semilunar valves (aortic stenosis). 

 Since this condition is associated with hypertrophy and dilatation 

 of the left ventricle, the slow emptying of the ventricle is partly due 

 to the greater quantity of blood which it contains. In whatever 

 way the delay in the emptying of the ventricle is brought about, the 

 most probable explanation of the anacrotic pulse is that the delay 

 affords time for one or more secondary waves to be developed in the 

 arterial system before the summit of the curve has been reached, and 

 that these are superposed upon the long-drawn primary elevation. 

 In aortic insufficiency, where the left side of the heart is nevar cut off 

 entirely from the aorta, the auricular impulse is sometimes marked 

 on the pulse-curve as a distinct elevation; and this gives rise to a 

 peculiar kind of anacrotic pulse, especially in the arteries nearest the 

 heart (Fig. 38, F, p. 103). 



Frequency of the Pulse. In health, the working of the cardiac 

 pump is so smooth and apparently so self-directed that it needs a 

 certain degree of attention to perceive that the rate of the stroke is 

 not absolutely constant. It is, in reality, affected by many internal 

 conditions and external influences. 



At the end of foetal life the rate is given as 144 to 133 ; from birth 

 till the end of the first year, 140 to 123 ; from 10 to 15 years, 91 to 76 ; 

 from 20 to 25 years, 73 to 69. It remains at this till 60 years, and 

 increases again somewhat in old age.* At all ages the pulse is some- 

 what quicker in the female than in the male, the excess amounting to 

 about 8 beats a minute. So that if we take the average rate for a 

 man (in the sitting position) as 72, the average for a woman will be 

 80. The difference is partly due to the fact that the average man 

 is taller than the average woman ; and it is known that in persons of 

 the same sex and age the pulse-rate has an inverse relation to the 

 stature. But there may be, in addition, a real sexual difference. 

 It must not be forgotten that a certain number of perfectly healthy 

 persons, who may even be noted for their powers of physical en- 

 durance, have an habitually slow pulse, not above 50 in the minute. 

 The position of the body exercises a slight, but relatively constant, 

 influence on the rate, which is greater in the standing than in the 

 sitting posture, and greater in the latter than in the recumbent 

 position. And this is true even when muscular action is as far as 

 possible eliminated by fastening the person to a board. The pulse 



* It must be remembered that these numbers are merely averages. Some 

 healthy individuals have a much lower pulse-rate than 72 per minute, and 

 some a rate considerably greater. Thus, while the average pulse-rate (taken 

 in the sitting position) of 87 healthy (male) students, whose ages ranged from 

 1 8 to 36 years, was 73, the extreme variation was from 54 to 89. In the 

 standing position the average was 80, and the variations 64 to 105. In 

 the supine position, average 69, and variations 48 to 98. After a short spell 

 of muscular exercise (generally running up and down some nights of stairs) 

 the average in the standing position was 119, the variations 75 to 164, and 

 the average increase 32. 



