236 ANIMAL PHYSIOLOGY. 
riety of causes, which it is not always possible to explain fully 
in each case, but owing doubtless in great part to variations ih 
the anatomical relations. 
The Causes of the Sounds of the Heart.—There is general agree- 
ment in the view that the second sound is owing to the closure 
of the semilunar valves of the aortic and pulmonary vessels; 
the former, owing to their greater tension in consequence of the 
higher blood-pressure in the aorta, taking much the larger share 
in the production of the sound, as may be ascertained by listen- 
ing over these vessels in the exposed heart. When these valves 
are hooked back, the second sound disappears, so that there can 
be no doubt that they bear some important relation to the cau- 
sation of the sound. 
In regard to the first sound of the heart the greatest diver- 
sity of opinion has prevailed and still continues to exist. The 
following among other views have been advocated by physi- 
ologists: 
1. The first sound is caused by the tension and vibration of 
the auriculo-ventricular valves. 
2. The first sound is owing to the contractions of the large 
mass of muscle composing the ventricles. 
3. The sound is directly traceable to eddies in the blood. 
In favor of the first view it was argued that by agreement 
the second sound was valvular, and why not the first >—And 
again that malformations of the valves gave rise to “murmurs” 
(“ bruits”’), which either obscured or replaced the true sound. 
The second opinion was supported by the fact that the larger 
the heart the more powerful the sound; that when the blood 
was cut off from the heart by ligature of the vessels success- 
ively, the sound could still be heard; that with fatty degenera- 
_ tion of the muscle-fibers of the heart, it had been found that 
the sound was weak—and similar arguments. 
Recently it has been contended very strongly that the first 
sound may be heard by a double stethoscope placed over an ex- 
cised, bloodless, mammalian heart, or even ventricle, while it 
still beats. 
The third opinion was less vigorously upheld, but certain 
experiments and physical phenomena were pointed to in sup- 
port of it. vi 
Against the arguments adduced above it may be stated that 
the first sound may be conceived as overpowered by a brut 
without being replaced by it in the proper sense of the word. 
It is well known that the cardiac muscle is peculiar, occupying 
