556 CIRCULATION OF BLOOD AND LYMPH. 



from the veins. Variations in the size of the blood-vessels, such 

 as 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 ob- 

 servers* 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 is 

 insufficient to distend the heart completely, but during muscular 

 work the increase in venous pressure and velocity distends the 

 heart, during diastole, to its maximal extent. 



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,f both tones, from a musical 

 standpoint, fall in the bass clef, and are separated by a musi- 

 cal 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 stethoscope is usually employed, and 

 this method of investigation by hearing is designated as auscultation. 

 The importance of these heart sounds in diagnosis was first em- 

 phasized 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 regard- 

 ing 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 ultimately 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. This view is made 

 probable by a number of experimental results, some of the most 

 important of which were brought out by Williams in a report (1836) 

 of a committee appointed by the British Association for the 

 special purpose of investigating the subject. It has been shown: 

 (1) That the second sound disappears before the first sound when 



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

 1912. 



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



