MOVEMENTS OF THE HEART. 139 



of their veins, the effect passing quickly forward ; the ventricles con- 

 tract more slowly, but simultaneously in every part. 



During each beat of the heart two sounds may be heard, followed by 

 a silence. The first sound is dull ; the second, which fol- Sounds of the 

 lows it quickly, is sharp. They may be imitated by artic- heart - 

 ulating the syllables lubb, dup. The first is due to the contraction 

 of the muscular fibres of the ventricles, and the striking of the apex of 

 the heart against the wall of the chest ; to a certain extent, the opening 

 of the semilunar valves, and the rush of the blood into the pulmonary 

 artery and aorta contribute to it. The second sound is due to the shut- 

 ting of the semilunar valves of the aorta and pulmonary artery. 



At each contraction of the ventricles the heart strikes against the walls 

 of the chest, usually between the fifth and sixth ribs, and an inch or two 

 to the left of the sternum. This motion is partly due to the action of 

 the spiral muscular fibres of the ventricles, which, gives a tilt to the 

 heart, and partly to the globular form which the whole organ suddenly 

 assumes. 



The number of pulsations made by the heart differs very much at dif- 

 ferent periods of life: at birth it is from 130 to 140 per Number of pui- 

 minute ; at the seventh year, from 80 to 85 ; during mature sations. 

 life, from 70 to 75 ; and in old age, from 50 to 65. In females it is 

 more frequent than in males. It observes a general relation with the 

 number of respirations, five pulsations commonly occurring during one 

 respiration. It varies with incidental circumstances. During sleep it 

 declines in frequency ; after eating, or during exercise, it is quickened. 

 Examined from morning to evening, it becomes slower by degrees. Ly- 

 ing down, the pulse is slower ; in a sitting posture, more frequent ; and 

 still more so when standing, the variations depending on muscular exer- 

 tion. In conditions of disease, the ratio between the number of pulsa- 

 tions and respirations is variable. 



The walls of the left ventricle are twice as thick as those of the right, 

 and the force of its contractions is about double. The ca- Structure and 

 pacity of the two ventricles is nearly the same, and is taken power of the 

 at about three ounces. The active force with which the au- walls ' 

 ricles dilate is feeble, and wholly incompetent to exert any thing like the 

 suction power at one time supposed, yet that they are not distended by 

 the mere influx of the blood is satisfactorily proved by their dilatation 

 after the heart has been cut out. 



With respect to the absolute force which the left ventricle exerts for 

 the propulsion of the blood into the systemic arteries, it is stated to be 

 13 Ibs. This result is derived from the consideration that the pressure 

 of the blood in the aorta is about 4 Ibs. 3 oz. 



That the motions of the heart can not be referred to the presence of the 



