THE CIRCULATION OF THE BLOOD. 317 



volume is accommodated by the forward movement of the blood as 

 a whole and by the sudden distention of that portion of the aorta 

 nearest the heart, both effects being due to the energy of the heart's 

 contraction. The distention of this first segment of the aorta is 

 almost immediately followed by a recoil of its elastic walls which, 

 through pressure on the blood, causes the next-lying segment to 

 expand; this in turn recoils and causes a third segment to expand, 

 and so on until the expansion has traveled not only over the aorta, 

 but over all its branches as well, as far as their final terminations. 



This expansion movement which thus passes from the beginning 

 to the end of the arterial system in the form of a wave is known as the 

 pulse-wave or pulse. Coincident with the expansion and recoil of the 

 arterial system there is a slight alternate increase and decrease of 

 the general blood-pressure, as shown by the small curves on a blood- 

 pressure tracing. 



The pulse-wave which thus spreads itself over the entire arterial 

 system with each systole of the heart can be perceived in certain 

 localities by the eye, by the sense of touch, and investigated with 

 various forms of apparatus or instrumental means. The pulse- 

 wave, or at least the elevation of the soft tissues overlying it, can be 

 seenjn the radial. artery, where it passes across the wrist- joint, in the 

 carotid artery, in the temporal artery, in the arteries of the retina under 

 certain conditions, with the ophthalmoscope. If the ends of the fingers 

 are firmly placed over the radial artery, not only the increase and de- 

 crease of pressure, but also many of the peculiarities of pulse-wave, may 

 be perceived. Without much difficulty it may be perceived that the 

 expansion takes place quickly, the recoil relatively slowly; that the 

 waves succeed one another with a certain frequency, corresponding 

 to the heart-beat; that the pulsations are rhythmic in character, etc. 

 Inasmuch as the individuality of the pulse-wave varies at different 

 periods of life and under different physiologic and pathologic condi- 

 tions, various terms more or less expressive have been suggested for 

 its varying peculiarities. Thus the pulse is said to be frequent or 

 infrequent according as it exceeds or falls short of a certain average 

 number 72 per minute; quick or slow, according to the suddenness 

 with which the expansion takes place or strikes the fingers; hard or 

 soft, tense or easily compressible, according to the resistance which 

 the vessel offers to its compression by the fingers; large, full, or small, 

 according to the volume of blood ejected into the aorta, or, in other 

 words, the degree of fullness of the arterial system. 



Frequency of the Pulse. As the pulse or the arterial expansion 

 is the direct result of the heart's action, its frequency must, under 

 physiologic conditions, coincide with that of the heart. All condi- 

 tions which modify the rate of the heart will modify at the same time 

 the rate of the pulse. (See page 275). 



