238 ENDOCARDIAC PRESSURE. [BOOK i. 



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. 



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. 



Endocardiac Pressure. 



131. Since the heart exists for the purpose of exerting pres- 

 sure on the blood within its cavities, by which pressure the circu- 

 lation of the blood is effected, the study of the characters of this 

 endocardiac pressure possesses great interest. Unfortunately 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 cannula, or tube open at the end, filled with 

 sodium carbonate solution, may be introduced into the jugular 

 vein and so slipped down into either the right auricle or the 

 right ventricle, or may be similarly introduced into the carotid 

 artery and with care slipped down through the aorta, past the 

 semilunar valves, into the left ventricle, and having been thus 

 introduced may, like the ordinary cannula used in studying 

 arterial pressure ( 115), be brought into connection with a mercury 

 manometer. In this way, as in the case of an artery, a graphic 

 record may be obtained of the changes of pressure taking place 

 in either of the above three cavities. But the changes in the 

 ventricular cavities are so great and rapid, that the inertia of the 

 mercury, an evil even in the case of an artery, comes so largely 

 into play that the curve described by the float on the mercury is 

 far from being an accurate record of the changes of pressure in the 

 cavity. 



The mercury manometer may however be made to yield 

 valuable results by adopting the ingenious contrivance of con- 

 verting the ordinary manometer into a maximum or a minimum 

 instrument. 



The principle of the maximum manometer, Fig. 36, consists in the 

 introduction into the tube leading from the heart to the mercury 

 column, of a (modified cup-and-ball) valve, opening, like the aortic 

 semilunar valves, easily from the heart, but closing firmly when fluid 

 attempts to return to the heart. The highest pressure is that which 

 drives the longest column of fluid past the valve, raising the mercury 



