THE CIRCULATION OF THE BLOOD. 



195 



when the ventricular systole commences, which puts the valves on the 

 stretch. The question whether this stretched condition of the valve 

 continues throughout the whole of the ventricular systole cannot be 

 definitely settled, but if it does not, the valvular element may possibly 

 take part in the production of the first part of the first sound only. It 

 is not unlikely too that the vibration of the ventricular walls themselves, 

 and of the aorta and pulmonary artery, all of which parts are suddenly 



DIASTOLE 



AURICLE 



VENTRICLE 



IMPULSE 



Fig. 165. Diagrammatic representation of the events of the cardiac cycle. For events which 

 occur in sequence, read in the direction of the curved arrow ; for synchronous events, read from 

 the centre to the periphery in any direction. (Coleman. ) 



put into a state of tension at the moment of ventricular contraction, 

 may have some part in producing the first sound. Secondly, the mus- 

 cular sound produced by contraction of the mass of muscular fibres 

 which form the ventricle. Looking upon the contraction of the heart 

 as a single contraction and not as a series of contractions or tetanus, it 

 is at first sight difficult to see why there should be any muscular sound 

 at all when the heart contracts, as contraction of a single muscle does 

 not produce sound. It has been suggested, however, that it arises from 

 the repeated unequal tension produced when the wave of muscular con- 

 tractions passes along the very intricately arranged fibres of the ventric- 



