142 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



It has been seen that the increased charge of blood which an artery receives 

 at the ventricular systole is accommodated partly by increased displacement of 

 blood toward the capillaries, and partly by that increase in the capacity of the 

 artery which is accompanied by the up-stroke of the pulse. The less the con- 

 tents of the artery the less is the arterial pressure, the less the tension of the 

 wall, and the more yielding is that wall. The more yielding the wall, the more 

 of the increased charge of blood does the artery accommodate by an increase of 

 capacity and the less by an increase of displacement. Therefore, a large pulse 

 often accompanies a low mean pressure in the arteries, and hence may appear 

 as a symptom after large losses of blood. In former days, when bloodletting 

 was practised as a remedial measure, imperfect knowledge of the mechanics 

 of the circulation sometimes caused life to be endangered ; for a "throbbing" 

 pulse in a patient who had been bled already was liable to be taken as an "in- 

 dication" for the letting of more blood. If this were done, an effect was 

 combated by repeating its cause. 1 



Celerity of Stroke. — When each up-stroke of the pulse appears to be 

 slowly accomplished, requiring a relatively long interval of time, the pulse 

 is called slow, or long. When each up-stroke appears to be quickly accom- 

 plished, requiring a relatively short time, the pulse is called quick or short. 

 These contrasted qualities are among the most obscure of those which the 

 skilled touch is called upon to appreciate. 



The Pulse-trace. — The rise and fall of a pulsating human artery, if near 

 enough to the skin, may be made to raise and lower the recording lever of a 

 somewhat complicated instrument called a sphygmograph. 2 Of this instru- 

 ment a number of varieties are in use. If the fine point of the lever be kept 

 in contact with a piece of smoked paper which is in uniform motion, a "pulse- 

 trace" or "pulse-curve" is inscribed, which shows successive fluctuations, 

 larger and smaller, which tend to be rhythmically repeated, and which depend 

 upon the movements of the arterial wall produced by the fluctuations of blood- 

 pressure. In an animal, a manometer may be connected with the interior of 

 an artery, and thus the fluctuations of the blood-pressure may be observed 

 more directly. It has been explained (p. 90) that the mercurial manometer 

 is of no value for the study of the finer characters of the pulse, owing to 

 the inertia of the mercury. On the other hand, the best forms of elastic 

 manometer give pulse-traces which are more reliable than those of the sphyg- 

 mograph. This is because the sphygmographic trace is subject to unavoid- 

 able errors dependent upon the physical qualities of the skin and other 

 parts which intervene between the instrument and the cavity of the artery. 

 Nevertheless, the sphygmographic pulse-trace, or " sphygmogram," is the 

 only pulse-trace which can be obtained from the human subject; and, when 

 obtained from an animal, it has so much in common with the trace recorded 

 by the elastic manometer, that the sphygmograph has been much used for the 

 Study of the human pulse, in health and disease, both by physiologists and by 



1 Marshall Hall: Researches principally relative to the Morbid and Ourative Effects of Loss of 

 Blood, London, 1830. 2 From a<pvyfi6c, pulse, and ypacpetv, to record. 



