244 



ENDOCARDIAC PRESSURE. 



[BOOK i. 



of the ventricle ; or we may use what is very nearly the same 

 thing, viz. a cardiographic tracing (Fig. 41), that is to say a tracing 

 of the cardiac impulse, which is a curve of changes in the pressure 

 exerted by the apex of the heart on the chest- wall. 



\\ \ \ 



\ b\6\c\ c\ 



WlWI/WWI/Wl^^ 



FIG. 40. (See Fig. 35.) 



Various forms of cardiograph have been used to record the cardiac 

 impulse. In some the pressure of the impulse, as in the sphygmograph, 

 is transmitted directly to a lever which writes upon a travelling surface. 

 In others the impulse is, by means of an ivory button, brought to bear 

 on an air-chamber, connected by a tube with a tambour as in Fig. 37 ; 

 the pressure of the cardiac impulse compresses the air in the air- 

 chamber, and through this the air in the chamber of the tambour by 

 which the lever is raised. In such delicate and complicated movements, 

 as those of the heart however, the use of long tubes filled with air is 

 liable to introduce various errors. 



We may begin our study of these curves at any point in the 

 cycle ; let it be the point V in Fig. 39. From this point the curve 

 rises very abruptly, almost in the vertical line, to a maximum at c; 

 and the same sudden large rise to a maxirnum occurs in the front-to- 

 back diameter of the ventricles (Fig. 40) and in the pressure of the 

 apex against the chest-wall (Fig. 41). There can be no doubt that 



FIG. 41. CAEDIOGKAM FKOM MAN. 



this corresponds to the first part of the systole of the ventricles. By 

 the sudden onset of the contraction of the ventricular fibres pressure 

 is brought to bear on the contents of the ventricle, and there being 

 as yet no escape for the blood, by the increasing contraction of the 



