134 ELEMENTARY PHYSIOLOGY 



more rapidly than it can be received by their branches. The 

 walls being elastic, they recoil on the blood, thus tending to force 

 it both back into the heart, and also into the smaller arteries. 

 But the backward pressure fills the pouches of the semilunar 

 valves, with which the arteries on both sides are provided, thus 

 preventing its motion in that direction ; hence we see that the 

 elastic recoil of the arteries assists the action of the heart in pro- 

 pelling the blood through the system. 



The whole of the action just described occupies less than one 

 second, and at every contraction of the ventricles about three 

 ounces of blood are forced into each of the two arterial systems. 



We may feel the beating of the heart by applying the hand 

 to the chest. This is due to the tilting forward of the apex of the 

 heart just at the commencement of the ventricular contraction, 

 and its consequent beating against the walls of the chest, between 

 the fifth and sixth ribs, and about two inches to the left of the 

 sternum (see fig. 77). 



The frequency of the beating varies considerably with the age, activity, 

 and various other circumstances. The average rates for persons of different 

 ages are as follows : 



During the first year . . . 120 per minute. 



In the adult 

 In old age 



second year, 

 third year . 

 seventh year 

 fourteenth year 



no , 

 95 



87 



85 to 80 , 

 80 to 70 , 

 70 to 60 , 



By applying the ear to the chest of another person we can hear the sounds 

 of the heart. For every beat there are two distinct sounds, followed by a 

 pause. The first sound is dull, and is supposed to be produced by the vibration 

 of the tricuspid and mitral valves, and also by the contraction of the walls of 

 the ventricles. The second sound is short and sharp, and is undoubtedly pro- 

 duced by the sudden closure of the semilunar valves. 



The pulse in the arteries is due to the sudden distension of 

 their walls immediately after each ventricular contraction. We 

 have already noticed that the blood is forced into the aorta by a 

 series of jerks, each corresponding with the contraction of the 

 left ventricle. Thus a rush of blood is produced in this vessel by 

 each contraction, and is transmitted by it into the smaller arteries. 

 Hence, if we cut an artery, the blood always issues in jerks. But 

 as the arterial walls are elastic, and are distended by the blood- 

 pressure, the recoil tends to diminish the suddenness of each jerk, 

 and, consequently, to convert the intermittent into a continuous 

 stream. Thus, if we cut a very small artery, we notice that the 

 jerks are prolonged considerably, so that the flow is almost con- 

 tinuous. 



