164 THE VASCULAR MECHANISM. 



the large veins are clamped so that no blood enters the ventricle, and, indeed, 

 may be recognized in the few beats given by a mammalian ventricle rapidly 

 cut out of the living body by an incision carried below the auriculo-ven- 

 tricular ring. Hence the view has been adopted that this first sound is a 

 muscular sound. In discussing the muscular sound of skeletal muscle (see 

 78), we saw reasons to distrust the view that this sound was generated by 

 the repeated individual simple contractions which made up the tetanus, and 

 hence correspond in tone to the number of those simple contractions re- 

 peated in a second, and to adopt the view that the sound was really due to a 

 repetition of unequal tensions occurring in a muscle during the contraction. 

 Now, the ventricular systole is undoubtedly a simple contraction, a prolonged 

 simple contraction, not a tetanus, and therefore under the old view of the 

 nature of a muscular sound, could not produce such a sound ; but, accepting 

 the other view, and reflecting how complex must be the course of the systolic 

 wave of contraction over the twisted fibres of the ventricle, we shall not find 

 great difficulty in supposing that that wave is capable in its progress of pro- 

 ducing such repetitions of unequal tensions as might give rise to a " muscular 

 sound," and consequently in regarding the first sound as mainly so caused. 

 Accepting such a view of the origin of the sound, we should expect to find 

 the tension of the muscular fibres, and so the nature of sound dependent on 

 the quantity of fluid present in the ventricular cavities, and hence modified 

 by ligature of the great veins, and still more by the total removal of the 

 auricles with the auriculo-ventricular valves. We may add that we should 

 expect to find it modified by the escape of blood from the ventricles into the 

 arteries during the systole itself, and might regard this as explaining why it 

 dies away before the ventricle has ceased to contract. 



Moreover, seeing that the auriculo-ventricular valves must be thrown into 

 sudden tension at the onset of the ventricular systole, which, as we have seen, 

 is developed with considerable rapidity, not far removed at all events from 

 the rapidity with which the semilunar valves are closed, a rapidity, there- 

 fore, 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 

 in some way with the sound of muscular origin, although the ear cannot 

 detect the mixture. 



If we accept this view, that the sound is of double origin, partly " muscu- 

 lar," 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 com- 

 pletely, altered and obscured in diseases of the auriculo-ventricular valves. 



Since the left ventricle forms the entire left apex of the heart, the mur- 

 murs 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 dis- 

 tinctly in the median line below the end of the sternum. 



Endocardiae Pressure. 



120. Since the heart exists for the purpose of exerting pressure on the 

 blood within its cavities, by which pressure the circulation of the blood is 

 effected, the study of the characters of this endocardiac pressure possesses 

 great interest. ITn fortunately, the observation of this pressure is attended 

 with great difficulties. The ordinary mercury manometer which is so useful 

 in studying the pressure in the arteries fails us when applied to the heart. 

 It is true that a long canula, or tube open at the end, filled with sodium 



