io8 THE CIRCULATION OF THE BLOOD AND LYMPH 



is further affected by the respiratory movements, especially when 

 they are exaggerated in forced breathing, being accelerated during 

 each inspiration (p. 287). It is also increased by the taking of food, 

 and especially of alcoholic stimulants, by muscular exercise, in fever 

 and many other pathological conditions, and by a high external 

 temperature. A warm bath, for example, causes a very distinct 

 acceleration of the heart; and Delaroche found that in air at the 

 temperature of 65 C. his pulse went up to 160. A cold bath may 

 depress the pulse-rate to 60, or even less. During sleep it may fall 

 to 50. It is greatly influenced by psychical events, and that in the 

 direction either of an increase or a decrease. Finally, it ought to be 

 remembered as of some practical importance that the pulse-rate in 

 women and children, but particularly in the latter, is less steady 

 than in men, and more apt to be affected by trivial causes. And it 

 is a good general rule to let a short interval elapse after the finger is 

 laid on the artery before beginning to count the pulse, so that the 

 acceleration due to the agitation of the patient may have time to 

 subside. 



Rate of Propagation of the Pulse- Wave. When pulse-tracings are 

 taken simultaneously at two points of the arterial system unequally 

 distant from the heart, by two sphygmographs whose writing-points 

 move in the same vertical straight line, it is found that the ascent 

 of the curve begins later at the more distant than at the nearer point. 

 Since waves like the pulse- wave travel with approximately the same 

 velocity in different parts of an elastic system like the arterial ' tree,' 

 this ' delay ' must be due to the difference in the length of the two 

 paths. The difference in length can be measured; the time- value of 

 the ' delay ' can be deduced from the rate of movement of the re- 

 cording surface; dividing the length by the time, we arrive at the 

 rate of propagation of the pulse-wave. The average rate has been 

 found to be about 7 metres per second in man in the arteries of the 

 upper limb, and 8 metres in those of the lower limb, the difference 

 being due to the smaller distensibility of the latter. In sleep the 

 velocity diminishes almost a metre a second. It increases in arterio- 

 sclerosis, where the distensibility of the arteries is diminished, and 

 in chronic nephritis with hypertrophy of the heart, in which the 

 blood-pressure is increased. The mean velocity of the pulse-wave 

 is about the same as the speed of a moderately fast steamship (say, 

 17 miles an hour), but less than that of a wave of the sea in a strong 

 gale. The velocity of the pulse- wave must not be confounded with 

 that of the blood-stream itself, which is not one-thirtieth as great. 

 A ripple passes over the surface of a river at its own rate a rate 

 that is independent of the velocity of the current. The passage of 

 the ripple is not a bodily transference of the particles of water of 

 which at any given moment the wave is composed, but the propaga- 

 tion of a change of relative position of the particles. The mere fact 



