THE CIRCULATION OF THE BLOOD AND LYMPH 89 



potential energy in the tense arterial wall, and this energy 

 is being continually transformed into work upon the blood 

 during the pause, the heart continuing, as it were, to con- 

 tract by proxy during its diastole. Thus, the blood pro- 

 gresses along the arteries in a series of waves, to which the 

 name of * blood-waves ' or ' pulse-waves ' may be given. 

 Wherever the pulse-wave spreads it manifests itself in 

 various ways by an increase of blood-pressure, an increase 

 in the mean velocity of the blood- flow, an increase in the 

 volume of organs, and by the visible and palpable signs to 

 which the name of pulse is commonly given in a restricted 

 sense. The intermittence in the flow with which the pulse- 

 wave is necessarily associated is at its height at the begin- 

 ning of the aorta. In middle-sized arteries, such as the 

 radial, it is still well marked, but it dies away as the capil- 

 laries are reached, and only under special conditions passes 

 on into the veins. 



The pulse was well known to the Greek physicians, and 

 used by them to a certain extent as an indication in practical 

 medicine. Harvey demonstrated with some clearness the 

 relation of the pulse to the contraction of the heart, but 

 Thomas Young was the first to form a proper conception of 

 it as the outward token of a wave propagated from heart to 

 periphery. 



When the finger is placed over a superficial artery like the 

 carotid, the radial or the temporal, a throb or beat is felt, 

 which, without measurement, seems to be exactly coinci- 

 dent with the cardiac impulse. In certain situations the 

 pulse can be seen as a distinct rhythmical rise and fall of 

 the skin over the vessel. The throbbing of the carotid, 

 especially after exertion, is familiar to everyone, and the 

 beat of the ulnar artery can be easily rendered visible by 

 extending the hand sharply on the wrist. When the pulse 

 is felt by the finger, it is not the expansion, but the 

 hardening of the wall of the vessel, due to the increase of 

 arterial pressure, that is perceived ; and even a superficial 

 artery when embedded in soft tissues so that it cannot be 

 compressed, gives no token of its presence to the sense of 

 touch. Sometimes an artery is longitudinally extended by 



