THE CIRCULATION OF THE BLOOD. 



285 



a 



f 



0' C'O 



FIG. 134. CARDIOGRAM, 



o, Au- 



a new volume of blood, in consequence of which the heart is pressing 

 against the chest walls. The systolic plateau is characterized by one or 

 more elevations and depressions, the true cause of which is unknown. 



From the correspondence of the curve of cardiac pressure against the 

 chest wall with the curve of intra-ventricular pressure it becomes possible 

 to indicate with approximate accuracy the time of the opening and closing 

 of the auriculo-ventricular valves and the 

 semilunar valves and hence the time of oc- 

 currence of the heart sounds and other fea- 

 tures of the cardiac cycle. Such a construc- 

 tion is shown in Fig. 134. 



Heart-sounds. Two sounds accompany 

 each pulsation of the heart, both of which 

 may be heard by applying the ear or the 

 stethoscope to the chest- walls, especially over 

 the region of the heart. One of these sounds 

 is low in pitch dull and prolonged; the other 



IS high in pitch, Clear and Short. 1 nese closing and opening of the auriculo- 



sounds can be approximately reproduced by ventricular valves; O', C', opening and 

 pronouncing the syllables lubb-dp, lubb- ttSttSSESsR 



dup. The long dull SOUnd OCCUrs With the ventricular discharge ;CC', time of the 



systole, the first phase of a new cardiac cycle, occurrence of the first and second 

 and is therefore termed the first sound; the ^ n ds respectively^ 

 short clear sound occurs at the beginning of the diastole, with the second 

 phase of the cardiac cycle, and is therefore termed the second sound. The 

 first sound is the systolic, the second the diastolic. With the ear it can 

 readily be determined that there is a brief pause between the first and second 

 sounds, and a longer pause between the second and the first sounds. The 

 duration of the first sound is almost equal to the duration of the systole viz., 

 0.3 second; the duration of the second sound is not more than o.i second. 

 The systolic sound is heard most distinctly over the body of the heart; the 

 diastolic sound is heard most distinctly in the neighborhood of the third rib 

 to the right of the sternum. 



The causes of the heart-sounds have enlisted the attention of clinicians 

 and physiologists for years, and many factors have been assigned for their 

 production. At present it is generally believed that the first sound is the 

 product of at least two, possibly three, factors: viz., the contraction of the 

 muscle walls of the ventricles, the simultaneous closure and subsequent 

 vibration of the tricuspid and mitral valves, and the sudden increase of 

 pressure of the apex of the heart against the chest-wall. 



That the contraction of the ventricular muscle gives rise to a sound is 

 certain from the fact that it is perceptible in an excised heart when the 

 cavities are free from blood and when the valves are prevented from closing. 

 The explanation of this sound is extremely difficult, as the contraction, 

 though prolonged, is not of the nature of a tetanus and therefore not charac- 

 terized by rapid variations of tension. The apex element may be eliminated 

 by placing the individual in the recumbent position. 



The second sound is the product of the simultaneous closure and subse- 

 quent vibration of the aortic and pulmonary valves which occur at the 



