THE HEART BEAT. 549 



dilatation of the small arteries or possibly loss of tone in the veins, 

 may bring about a condition of venous stasis and cut down the 

 supply of blood to the heart on the venous side. Two observers* 

 who have studied by indirect means the output of blood from the 

 right ventricle in man state that the volume of blood discharged 

 may vary from 2.8 liters per minute during rest to as much as 21.6 

 liters during muscular work. They conclude that this great range 

 of output is governed by variations in the venous filling of the heart 

 during diastole. At rest the venous inflow distends the heart dur- 

 ing diastole to a certain volume. During muscular work there is 

 a much greater flow of venous blood into the right side of the 

 heart. Consequently the ventricle will be more distended or will 

 contain a larger charge of blood and a corresponding increase in 

 outflow will occur during the systole. 



The Heart Sounds. — An interesting and important feature 

 of the heart beat is the occurrence of the heart sounds. Two 

 sounds are usually described, one at the beginning, the other 

 at the end, of the ventricular systole. The first sound has a deeper 

 pitch and is longer than the second, and their relative pitch and 

 duration are represented frequently by the syllables lubb-dup. 

 According to Haycraft,t both tones, from a musical standpoint, 

 fall in the bass clef, and are separated by a musical interval of a 

 minor third. The sounds are readily heard by applying the ear 

 to the thorax over the heart, but for diagnostic purposes the stetho- 

 scope is usually employed, and this method of investigation by 

 hearing is designated as auscultation. The importance of these 

 heart sounds in diagnosis was first emphasized by Laennec (1819), 

 and since his time a great number of theories have been proposed 

 to explain their causation. Indeed, the subject is not yet closed, 

 although certain general views regarding their cause and the time 

 of their occurrence are generally accepted. The second sound is 

 found to follow immediately upon the closure of the semilunar 

 valves. The usual view, therefore, is that the sound is due ulti- 

 mately to the vibrations set up in these valves by their sudden 

 closure. These vibrations are transmitted to the column of blood 

 in the aorta (or pulmonary artery) and then to the intervening 

 tissue of the chest wall. 



The physician uses this view of the cause of the second sound in 

 auscultation, and it is evident that the nature of the sound or its 

 replacement by murmurs will give useful testimony regarding the 

 condition of the semilunar valves. The first heart sound has of- 

 fered more difficulty. It occurs at or shortly before the closure of 

 the auriculoventricular valves, and it would seem natural, there- 



* Krogh and Lindhard, "Skandinavisches Archiv f. Physiologic," 27, 100, 

 1912. 



t "Journal of Physiology," 11, 486, 1890. 



