Chap. iv.J THE VASCULAR MECHANISM. 191 



semilunar valves are closed, a rapidity, therefore, capable of giving 

 rise to vibrations of the valves adequate to produce a sound, it is 

 difficult to escape the conclusion that the closure of these valves 

 must also generate a sound, which in a normally beating heart is 

 mingled with the sound of muscular origin. 



If we accept this view that the sound is of double origin, 

 partly ' muscular,' partly ' valvular/ both causes being dependent 

 on the tension of the ventricular cavities, we can perhaps more 

 easily understand how it is that the normal first sound is at times 

 so largely, indeed, we may say so completely altered and obscured 

 in diseases of the auriculo-ventricular valves, and how it may also 

 be modified in character by changes, such as hypertrophy, of the 

 muscular walls. 



Since the left ventricle forms the entire left apex of the 

 heart, the murmurs or other changes of the first sound heard most 

 distinctly at the spot of cardiac impulse belong to the mitral valve 

 of the left ventricle. Murmurs generated in the tricuspid valve 

 of the right ventricle are heard more distinctly in the median line 

 below the end of the sternum. 



§ 113. Endocardiac Pressure. Since it is the pressure exerted 

 upon the contents of the ventricle by the contraction of the 

 ventricular walls which drives the blood from the heart into the 

 aorta, and so maintains the circulation, the Study of this pressure, 

 endocardiac pressure, is of great importance. The mercurial 

 manometer, so useful in a general way in the study of arterial 

 pressure, is unsuited for the study of endocardiac pressure, since 

 the great inertia of the mercury prevents the instrument respond- 

 ing properly to the exceedingly rapid changes of pressure which 

 take place in the heart. We are obliged to have recourse to other 

 instruments. 



One method, having been used by Chauveau and Marey in 

 researches which have become ' classic,' deserves to be noticed, 

 though it is not now employed. It consists in introducing, in a 

 large animal, such as a horse, through a blood vessel into a cavity 

 of the heart, a tube ending in an elastic bag, Fig. 37 A, both tube 

 and bag being filled with air, and the tube being connected with 

 a recording ' tambour.' 



A tube of appropriate curvature, A. b. Fig. 37, is furnished at its 

 end with an elastic bag or 'ampulla' a. Such an instrument is 

 spoken of as a ' cardiac sound.' When it is desired to explore simul- 

 taneously both auricle and ventricle, the sound is furnished with two 

 ampulla?, one at the extreme end and the other at such a distance that 

 when the former is within the cavity of the ventricle the latter is 

 within the cavity of the auricle. Each ' ampulla ' communicates 

 by a separate air-tight tube with an air-tight tambour (Fig. 37 B) 

 on which a lever rests, so that any pressure on the ampulla is 

 communicated to the cavity of its respective tambour, the lever of 



