552 CIRCULATION OF BLOOD AND LYMPH. 



somewhat less of the ventricular charge is left in the heart after 

 systole, when the heart is beating at the normal rate (90), and 

 the quantity of blood discharged from the left ventricle at each 

 systole is approximately .002 of the body weight. It is evident 

 that when the aortic pressure rises to an abnormal level the 

 discharge of blood from the left ventricle will be or may be 

 diminished, with the result that the blood backs up in the left 

 auricle, thus raising the venous pressure in the lungs and retard- 

 ing the pulmonary circulation. On the other hand, as Hender- 

 son has especially emphasized, the outflow from the ventricle 

 must be influenced very directly by the inflow into the auricle 

 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 Hay craft, 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 



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

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



