316 CIRCULATORY SYSTEM. 



Papillary Muscles. It has been stated that during the ven- 

 tricular systole the heart shortens. It is manifest that unless 

 some provision was made this change in the shape of the heart 

 would permit of regurgitation of the blood into the auricles, 

 and thus would result a damming back of the blood in the venae 

 cavse and pulmonary veins ; for if the chordae tendineae were of 

 just the right length at the beginning of the ventricular systole 

 to keep the segments of the mitral and tricuspid valves so exactly 

 in place as not to permit a leakage, then when the ventricles 

 shortened these cords would be too long, and would permit the 

 segments to enter the cavity of the auricles and there separate, 

 leaving a considerable gap through which the blood could pass. 

 That this does not occur is due to the papillary muscles. As the 

 ventricles shorten, these structures contract sufficiently to take up 

 the slack in the cords, and keep them just long enough to main- 

 tain the proper approximation of the segments of the valves. 



Cardiac Sounds. When the ear is placed against the chest 

 wall in the region of the heart two sounds are heard during each 

 cardiac period. The first of these sounds is heard loudest that 

 is, at its maximum of intensity over the apex, and is by some 

 writers called the apex-sound. For the reason that it is the first 

 sound heard after the pause it is called the first sound, and because 

 it occurs at the beginning of the systole of the heart (ventricular 

 systole) it is called the systolic sound. The second sound is heard 

 loudest over the base of the heart, and is therefore sometimes de- 

 scribed as the basic sound; inasmuch as it occurs during the 

 diastole, it has received the name of the diastolic sound. More 

 commonly, however, it is spoken of as the second sound. 



Characteristics of the Cardiac Sounds. The first sound, as com- 

 pared with the second, is lower in pitch and longer in duration, 

 and has been likened to the sound of the word liibb. The second 

 sound is higher in pitch and shorter in duration than the first, and 

 has been likened to the sound of dup. These sounds occur suc- 

 cessively, without any interval between them ; in the pause which 

 follows no sound is heard. 



Causes of the Cardiac Sounds. The cause of the second sound 

 is undoubtedly the closure of the aortic and pulmonary valves. 

 This has been demonstrated by hooking back the segments of the 

 valves, when the sound disappears, to reappear when the seg- 

 ments are set free. The causation of the first sound is not so 

 simple ; indeed, authorities are not at one on this point. The 

 closure of the mitral and tricuspid valves contributes something 

 to it, but the closing of the valves is not the sole factor, for in a 

 heart in which there is no blood the sound may still be heard, 

 although modified, and in such a heart the valves would not close. 

 The contraction of the muscular tissue of the heart gives forth a 

 sound, as does indeed the contraction of other muscles, and this 



