202 THE MECHANISM OF THE BEAT. [BOOK i. 



In the instrument of Roy and Adaini, by an ingenious arrangement 

 into the details pf which we need not go, a delicate rod placed horizon- 

 tally in connection with two points of the surface of the heart, of the 

 ventricles, for instance, as it glides to and fro, according as the two 

 points approach or recede from each other, records its movements by 

 means of a light lever. 



We give in Fig. 47 such a myocardiographic tracing, as it 

 is called ; the rise of the lever indicates an 

 approach, the fall a receding of two points 

 taken transversely across the ventricle of a 

 dog. 



What conclusions can we draw from the 

 features of the various curves which we have 

 given ? We have reproduced in some cases 

 more than one curve representing the same 

 event, for the important reason that certain 

 FIG. 47. MYOCARDIO- of the features of almost every curve are 



due > to some extent at least ' to the instru- 

 ment itself, and must not be taken as exact 

 records of what is actually taking place in the heart ; the inertia 

 of one or other part of this or that instrument used plays a more 

 or less important part in determining the form of the curve. It 

 will therefore be readily understood that the interpretation of 

 various heart curves is attended with great difficulties, and has 

 led to much discussion. We must content ourselves here with 

 confining our attention to the more important points, leaving many 

 details, however interesting, on one side. 



Let us begin with the beginning of the ventricular systole. 

 All the curves, curve of endocardiac pressure, cardiogram, myocar- 

 diogram, and others, shew the important fact that the systole begins 

 suddenly and increases swiftly until it reaches the beginning of 

 what we have called the " systolic plateau," c in Figs. 38, 39, 45, 

 3 in Fig. 46, d in Fig. 47. 



In some curves, as in Figs. 38, 39 B, 42, the rise is unbroken ; 

 in others, as in Figs. 39 A, 45, the rise is marked with a shoulder. 

 In Fig. 47, this shoulder b has been interpreted, by those who 

 maintain that papillary muscles begin their contraction later than 

 the main ventricular wall, as indicating that event. We will not 

 discuss the question here. 



In some of the pressure curves, as in Fig. 38, the rise of pressure 

 in the ventricle due to the actual systole is preceded by & slight 

 temporary rise. This has been interpreted as indicating a slight 

 rise of pressure in the ventricle due to the auricular systole just 

 preceding the ventricular systole ; but this interpretation has been 

 debated, and indeed the slight rise in question is not always seen. 

 Similarly, some curves shew a gradual but very slight increase of 

 pressure in the ventricle during the preceding diastole ; this has 

 been interpreted as indicating a rise of pressure due to the gradual 



