THE AUTERIAL CI RCUL ATTON. 331 



and prevent any movement in a backward direction. The blood is thus 

 urged onward, under the pressure of the arterial elasticity, into the 

 capillary system. When the arteries have become partially emptied, 

 and have returned to their previous dimensions, they are again dis- 

 tended by another contraction of the heart. In this manner a succes- 

 sion of expansions is produced, which can be felt throughout the body 

 wherever the arterial ramifications penetrate. This phenomenon is 

 known by the name of the arterial pulse. 



Since each arterial expansion is produced by a ventricular systole, 

 the pulse, as felt in any superficial artery, is a convenient guide for 

 ascertaining the frequency and regularity of the heart's action. The 

 radial artery at the wrist, owing to its easily accessible situation, is 

 mainly employed for this purpose. Any increase or diminution in the 

 frequency of the heart's action is accompanied by a similar change in 

 the arterial pulsations ; and alterations in the force or regularity of the 

 cardiac movements are also indicated by corresponding modifications 

 of the pulse at the wrist. 



The average frequency of the pulse in the human subject is, for the 

 adult male in a state of quiescence, 70 beats per minute. This rate 

 may be more or less accelerated by any muscular exertion. Even the 

 difference of muscular effort between the positions of standing, sitting, 

 and lying down, will make a normal difference in the pulse of from 8 to 

 10 beats per minute. Age has a very marked influence on the rapidity 

 of the pulse ; it being found, as a rule, more rapid the younger the sub- 

 ject of observation. According to Dr. Carpenter, the pulse of the 

 foetus, before birth, 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 only reduced to the adult standard 

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

 duce a temporary acceleration of the pulse, varying in degree with the 

 peculiarities of the individual. 



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

 to its force ; that is, a slow pulse, within physiological limits, is a strong 

 one ; a rapid pulse is a feeble one. This is readily noticeable in ex- 

 periments upon the lower animals, where the force of the heart's action 

 may be measured by the arterial impulse ; and where an increase in the 

 frequency of the cardiac pulsations is almost invariably accompanied 

 by a diminution in their strength. The same thing is true in cases of 

 increased frequency of the heart's action from morbid causes ; the pulse 

 in febrile or chronic affections becoming weaker as it growls more rapid. 

 An excessive rapidity of the pulse is an indication of great danger ; 

 and, in the adult male, a continuous pulse of 160 per minute is almost 

 invariably a fatal symptom. 



Increased Curvature of the Arteries in Pulsation. When the blood 

 is driven by the ventricular systole into the arteries, these vessels are 

 not only distended laterally, but are elongated as well as widened, 



