A SINGLE CONTRACTION OF THE HEART 



107 



features closely analogous to those of a simple muscle twitch. The 

 great difference lies in the time occupied. The time measurements 

 are seen to be — 



(a) The latent period, 0117 sec. 



(b) The period of contraction, 

 0-767 sec. 



(c) The period of relaxation, T067 

 sec. 



In examining the general form of 

 the curve it is seen that the top tends 

 to be flattened ; a condition which is 

 often more prominent than in this 

 particular curve (see curves of fig. 91). 

 In different hearts the form of the 

 curve varies considerably, differences 

 which are largely due to the different 

 temperatures at which they are ex- 

 amined, and secondly to the time of 

 the year. The flattening of the top 

 of the curve tends to show that the 

 contraction wave possesses so slow 

 a rate that the first fibres affected 

 return to rest before the wave has 

 extended over all the heart fibres. 

 This follows from the same considera- 

 tions which we have already gone 

 into when discussing the features 

 of the contraction wave in muscle 

 (p. 73). 



In figs. 89 and 90 we have two 

 further tracings taken from different 

 hearts, and with the recording surface 

 moving at different rates. In these 

 figures the relaxation of the heart is 

 seen to be more rapid than the con- 

 traction ; a condition which is com- 

 moner than the reverse effect recorded 

 in fig. 88. In both these figures the 

 first and major part of the relaxation 

 is most rapid ; it then becomes much 

 slower and gradually ceases. Note 

 that the apex of each curve is very blunt, even flattened, indicating 

 that the contraction is sustained for some time. 



