The Blood and its Circulation 



both seen and felt on the left side. The heart beat is unu- 

 sually strong during active bodily exertion and under mental 

 excitement. This is commonly known as palpitation. 

 ^VThe impulse of the heart is due to the striking of the 

 lower, tense part of the ventricles the apex of the heart 

 against the chest wall at 

 the moment of their vigor- 

 ous contraction. 

 ^c 230. The Sounds of the 

 Heart. If the ear be applied 

 over the region of the heart, 

 two distinct sounds will be 

 heard. Their character may 

 be tolerably imitated by pro- 

 nouncing the syllables "lub," 

 " dup." One sound is heard 

 immediately after the other, 

 then there is a pause, then 

 come the two sounds again. 

 The first is a dull, muffled 

 sound, known as the " first 

 sound," followed at once by 

 a shorter and sharper sound, 



FIG. 78. Muscular Fibers of 

 the Ventricles. 



A, superficial fibers common to both ven- 

 tricles; B, fibers of the left ventricle; 

 C, deep fibers passing upwards towards 

 the base of the heart ; D, fibers penetrat- 

 ing the left ventricle. 



known as the "second 

 sound" of the heart. 



The precise cause of the 

 first sound is not certainly 



known, but the second sound is, without doubt, caused by 

 the sudden closure of the semilunar valves of the pul- 

 monary artery and the aorta, at the moment when the 

 contraction of the ventricles is completed. 

 ><^2.31. The Nervous Control of the Heart. The regular, 

 rhythmic movement of the heart is maintained by the action 



