CHAP. XI.] THE VASCULAR SYSTEM. 133 



The radial artery at the wrist, owing to its accessible situation, 

 is usually employed for this purpose. Any variation in the 

 frequency, force, or regularity of the heart's action is indicated 

 by a corresponding modification of the pulse at the wrist. 



The average frequency of the pulse in man is seventy-two 

 beats per minute. This rate may be increased by muscular 

 action. Even the variation of muscular effort entailed between 

 the standing, sitting, and recumbent positions will make a 

 difference in the frequency of the pulse of from eight to ten 

 beats per minute. Age has a marked influence in the same 

 direction. According to Carpenter, the pulse of the foetus is 

 about 140, and that of the newly born infant 130. During the 

 first, second, and third years, it gradually falls to 100 ; by the 

 fourteenth year to 80 ; and is reduced to the adult standard by 

 the twenty-first year. At every age, mental excitement may 

 produce a temporary acceleration, varying in degree with the 

 peculiarities of the individual. 



As a rule, the rapidity of the heart's action is in inverse ratio 

 to its force. A slow pulse, within physiological limits, is 

 usually a strong one, and a rapid pulse comparatively feeble. 

 The same is true in disturbance of the heart's action in disease ; 

 the pulse in fever, or other debilitating affections becoming 

 weaker as it grows more rapid. 



Arterial tension. When an artery is severed, the flow of 

 blood from the proximal end (that on the heart side) comes in 

 jets corresponding to the heart-beats, though the flow does not 

 cease between the beats. The larger the artery, and the nearer 

 to the heart, the greater the force with which the blood issues, 

 and the more marked the remittance of the flow. 



When a corresponding vein is severed, the flow of blood, 

 which is chiefly from the distal end (that away from the heart), 

 is not remittent, but continuous ; the blood comes out with 

 comparatively little force, and " wells up," rather than " spurts 

 out." 



The continuous uninterrupted flow of blood in the veins is 

 caused by the elasticity of the arterial walls. On account of 

 the small size of the capillaries and small arteries the blood 

 meets with a great deal of resistance in passing through them ; 

 and, in consequence, the blood cannot get through the capilla- 

 ries into the veins so rapidly as it is thrown into the arteries by 



