102 THE CIRCULATION OF THE BLOOD AND LYMPH 



and driving the blood on into the capillaries. The work done by the 

 ventricle is, in fact, partly stored up as 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 contract by proxy during its diastole. Thus, the blood 

 progresses 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 beginning of the aorta. 

 In middle-sized arteries, such as the radial, it is still well marked, but 

 it dies away as the capillaries are reached, and only under special 

 conditions passes on into the veins, where, however, as has just been 

 mentioned, pulsatory phenomena of a different origin maybe detected. 



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 prop- 

 agated 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 coincident 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 extend- 

 ing 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 the 

 pulse-wave, and this extension may be far more conspicuous than 

 the lateral dilatation. This is particularly seen when one point of 

 the vessel is fixed and a more distal point offers some obstruction to 

 the blood- flow, as at a bifurcation or in an artery which has been 

 ligatured and divided. 



By means of the sphygmograph, the lateral movements of the 

 arterial wall, or, rather, in man, the movements of the skin and other 

 tissues lying over the bloodvessel, can be magnified and recorded. 



It would be very unprofitable to enumerate all the sphygmographs 

 which ingenuity has invented and found names for. The first attempt 



