220 THE CIRCULATION OF THE BLOOD 



The Tone of the Heart 



Since the diastolic volume of the heart has been found to become altered 

 under certain conditions, such as a change in C H of the blood, it has been 

 common to assume it possesses tone like that exhibited by skeletal muscle. 

 This however is not the case, for there is but one thing which determines 

 the distensibility of the heart during diastole, namely the pressure under 

 which the blood is flowing into it from the great veins. When this is con- 

 stant, the ventricle always dilates to the same degree. Changes in C H act 

 solely by altering the duration of systole and diastole, an increase pro- 

 longing the latter and a decrease, the former. 



