THE HEART 261 



gaining power to work again. Hence, after an effective 

 stimulus has been administered nothing is gained by 

 repetition until diastole has set in. The heart is said 

 to be refractory toward stimulation while it is in its 

 systolic phase. 



The Pulse. What we call the pulse is a momentary 

 swelling which we notice in an artery. It is associated 

 with the intermittent movement of the blood-stream 

 discussed in the previous chapter, but it stands in such 

 a relation to the heart action that consideration has been 

 postponed until now. The swelling which is felt occurs 

 after each systole of the left ventricle. It is the sign of 

 the transient increase of arterial content due to the 

 introduction of blood from the heart. The lateral en- 

 largement occurs first in the aorta, but is shifted with 

 great speed to all the branches of the arterial tree. In 

 thus shifting along the walls of the elastic vessels the 

 pulse has a wave-like character. 



On the surface of a river the ripples may run upstream, 

 across, or downstream and at rates entirely unrelated 

 to that of the current. So, in the case of the pulse, we 

 have to recognize that it travels at a speed which is of 

 an utterly different order from that of the blood-stream. 

 In fact the pulse is propagated many times faster than 

 the blood. A corpuscle may take four or five seconds to 

 run from the semilunar valves to the wrist, but the wave 

 of swelling will pass over the same distance in a fraction 

 of one second. 



The physician who feels the pulse with his practised 

 fingers can learn from it not only the frequency and 

 regularity of the heart-beat, but whether the output at 

 each systole is large or small and whether the prevail- 

 ing average pressure is high or low. Still other signs 

 of the condition of the circulation may be apparent to 

 him. If, for example, the aortic semilunar valves do 

 not close effectively, the pulse is said to have a " collaps- 

 ing" character, the swelling disappears and the tension 

 is eased with abnormal abruptness. 



