272 THE CIRCULATION. 



the semilunar valves, and in the action of the heart the shutting 

 back of the two sets of valves alternate with each other precisely 

 as do the first and second sounds ; and there is every probability, 

 to say the least, that the sudden tension of the valvular fibres pro- 

 duces a similar effect in each instance. 



2d. The first sound is heard most distinctly over the anterior 

 surface of the ventricles, where the tendinous cords supporting the 

 auriculo- ventricular valves are inserted, and where the sound pro- 

 duced by the tension of these valves would be most readily con- 

 ducted to the ear. 



3d. There is no reason to believe that the current of blood 

 through the cardiac orifices could give rise to an appreciable sound, 

 so long as these orifices, and the cavities to which they lead, have 

 their normal dimensions. An unnatural souffle may indeed origi- 

 nate from this cause when the orifices of the heart are diminished 

 in size, as by calcareous or fibrinous deposits; and it may also 

 occur in cases of aneurism. A souffle may even be produced at 

 will in any one of the large arteries by pressing firmly upon it 

 with the end of a stethoscope, so as to diminish its calibre. But in 

 all these instances, the abnormal sound occurs only in consequence 

 of a disturbance in the natural relation existing between the volume 

 of the blood and the size of the orifice through which it passes. 

 In the healthy heart, the different orifices of the organ are in exact 

 proportion to the quantity of the circulating blood ; and there is no 

 more reason for believing that its passage should give rise to a 

 sound in the cardiac cavities than in the larger arteries or veins. 



4th. The difference in character between the two sounds of the 

 heart depends, in all probability, on the different arrangement of 

 the two sets of valves. The second sound is short, sharp, and dis- 

 tinct, because the semilunar valves are short and narrow, superficial 

 in their situation, and supported by the highly elastic, dense and 

 fibrous bases of the aortic and pulmonary arteries. The first sound 

 is dull and prolonged, because the auriculo- ventricular valves are 

 broad and deep-seated, and are attached, by their long chordas 

 tendineae to the comparatively soft and yielding fleshy columns of 

 the heart. The difference between the first and second sounds can, 

 in fact, be easily imitated, by simply snapping between the fingers 

 two pieces of tape or ribbon, of the same texture but of different 

 lengths. (Fig. 84.) The short one will give out a distinct and sharp 

 sound ; the long one a comparatively dull and prolonged sound. 



Together with the first sound of the heart there is also to be 



