48 CIRCULATION OF THE BLOOD ACTION OF THE HEART. 



ute to its production." It will be assumed, therefore, that the sounds of the 

 heart have a mechanism that may be summarized as follows : 



The first sound of the heart is a compound sound. It is produced by the 

 closure of the auriculo- ventricular valves at the beginning of the ventricular 

 systole, to which are superadded, the muscular sound, due to the contraction 

 of the muscular fibres of the heart, and the impulsion-sound, due to the 

 striking of the heart against the walls of the thorax. 



The second sound is a simple sound. It is produced by the sudden clos- 

 ure of the aortic and pulmonic semilunar valves, immediately following the 

 ventricular systole. 



It is of importance, with reference to pathology, to have a clear idea of 

 the currents of blood through the heart, with their exact relations to the 

 sounds and intervals. At the beginning of the first sound, the blood is for- 

 cibly thrown from the ventricles into the pulmonary artery on the right side 

 and the aorta on the left, and the auriculo-ventricular valves are closed. 

 During the period occupied by this sound, the blood is flowing through the 

 arterial orifices, and the auricles are receiving blood slowly from the vense 

 cavse and the pulmonary veins. When the second sound occurs, the ventri- 

 cles having become relaxed, the recoil of the arterial walls, acting upon the 

 column of blood, immediately closes the semilunar valves upon the two sides. 

 The auricles continue to dilate, and the ventricles are slowly receiving blood. 

 Immediately following the second sound, during the first part of the interval, 

 the auricles become fully dilated ; and in the last part of the interval, imme- 

 diately preceding the first sound, the auricles contract and the ventricles are 

 fully dilated, This completes a single revolution of the heart. 



Frequency of the Heart's Action. The number of pulsations of the heart 

 is not far from seventy per minute in an adult male and is between seventy 

 and eighty in the female. There are individual cases, however, in which the 

 pulse is normally much slower or more frequent than this, a fact which must 

 be remembered when examining the pulse in disease. It is said that the 

 pulse of Napoleon I. was only forty per minute. Dunglison mentioned a 

 case which came under his own observation, in which the pulse presented an 

 average of thirty-six per minute. The same author stated that the pulse of 

 Sir William Congreve was never less than one hundred and twenty-eight per 

 minute, in health. It is by no means unfrequent to find a healthy pulse of a 

 hundred or more a minute ; but in the cases reported in which the pulse 

 has been found to be forty or less, it is possible that every alternate beat of 

 the heart was so feeble as to produce no perceptible arterial pulsation. In 

 such instances, the fact may be ascertained by listening to the heart while 

 the finger is placed upon the artery. 



Influence of Age and Sex. In both the male and female, observers have 

 constantly found a great difference in the rapidity of the heart's action at 

 different periods of life. The pulsations of the heart in the foetus are about 

 140 per minute. At birth the pulse is 136. It gradually diminishes during 

 the first year to about 128. The second year, the diminution is quite rapid, 

 107 being the mean frequency at two years of age. After the second year, 



