CHAP. XXVIII.] THE CIRCULATION IN THE ARTERIES. 353 



systole, the curves are distinctly altered so as to form segments of 

 larger circles, a motion is communicated to the artery, and a 

 change of place results; and straight arteries, which are more 

 or less confined by the superjacent parts, become slightly curved 

 under the same force. Thus, in the course of time, the arteries, 

 especially those of parts to which, by reason of a more active 

 nutrition in them, there is a considerable afflux of blood, assume a 

 tortuous form, as may be seen in the temporal and radial arteries 

 of old persons, and in the spermatic artery of the bull. 



The Pulse. When the finger is applied to an artery during life, 

 it is felt to beat or pulsate in correspondence with the systolic 

 actions of the heart, so that the number of pulsations in the artery 

 corresponds exactly with the number of beats of the heart, and if 

 an occasional interruption in the heart's action take place, or what 

 is called an intermission, there will be at the same time a failure 

 in the beats in the artery. 



This phenomenon is called the pulse. From their contiguity to 

 the heart, it is always present in arteries, but it may occur in 

 veins under circumstances to be explained hereafter. It is due to 

 the same cause which occasions the blood to flow per saltum, or by 

 successive jets, from a divided artery. That cause arises out of the 

 manner in which blood is pumped into the arterial system by suc- 

 cessive jerks. Each jet of blood creates a wave which moves along 

 the whole arterial system. The same phenomenon may be observed 

 if water be injected by successive jerks into a narrow channel 

 already full or nearly full, and open on the surface. Each fresh 

 jet will create on the surface of the water in the channel, a wave, 

 which may be followed to its most distant extremity. This wave, 

 even in a rigid tube, if sufficiently forcible, would communicate to 

 the wall of the tube a thrill or vibration indicating the course which 

 the wave takes. But in an elastic tube, which yields under the 

 injecting force, the phenomenon is more distinctly perceived as the 

 tube dilates under the pressure of the advancing wave. 



It is important to notice, that the phenomenon of the pulse in 

 arteries, is due solely to the wave excited by each successive injection 



is carried on in the finer capillaries, with an almost even tenor of velocity, in 

 the same manner as the spouting water of some fire-engines is contrived to flow 

 with a more even velocity, notwithstanding the alternate systoles and diastoles 

 of the rising and falling embolus or force ; and this, by the means of a large in- 

 verted globe, wherein the compressed air, alternately dilating or contracting, 

 in conformity to the workings to and fro of the embolus, and thereby impelling 

 the water more equally than the embolus alone would do, pushes it out in a 

 more nearly equal spout." Haemastatics, p. 22, 26. 



