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EXPERIMENTAL PHYSIOLOGY 



chronograph tracing of a spring vibrating two, four, or eight times per 

 second (fig. 29). Having recorded several beats, change the rate of 

 the drum to h cm. per second, and record again. The rate may still 

 further be reduced and so a series of tracings produced. 



Fig. 84 shows some typical tracings taken by this method, but 

 from different hearts. In I great care had been taken to avoid any 

 loss of blood during the preparation of the heart. We see that there 



Fig. 84 



-Becord of the Movements of the Frog's Heart by the Suspension 

 Method. Time Tracing, 4 per sec. 



are four ascents on the curve, viz. (i.) at a ; (ii.) from b to c ; (iii.) from 

 d to e ; and (iv.) from e to /. The little notch at a is due to the sinus 

 contraction. It is never very marked, and commonly is absent 

 altogether. This is due to the position of the sinus, which does not 

 allow it to produce much eiFect upon the recording lever ; and secondly, 

 because it usually occurs either during the relaxation, or even at the 

 end of the ventricular contraction, and is therefore masked by the 

 greater movement due to them. Even in this instance we see that 



