HEART 397 



the heart, the auricular and ventricular systoles do not vary, 

 but in a rapidly acting heart the pause is short, in a slowly 

 acting heart it is long. Taking the ordinary human heart rate 

 of 72 per minute, the auricular systole lasts for one-eighth 

 of the whole cardiac cycle, the ventricular for three-eighths, 

 and the pause for four-eighths. 



4. Changes in the Shape of the Chambers. 



1. Auricles. — These simply become smaller in all 

 directions during systole. 



2. Ventricles. — The changes in the diameters of the 

 ventricles may be studied by fixing them in the various 

 phases of contraction and measuring the alterations in 

 the various diameters. 



The shape in diastole when the muscular fibres are 

 relaxed is determined by the fibrous pericardium which 

 surrounds the heart, and by the position of the body, 

 the force of gravity leading to the expansion of the 

 ventricles at their dependent part. The condition at the 

 end of systole may be studied by rapidly excising the heart 

 while it is still beating, and plunging it in some hot solution 

 to fix its contraction. 



The condition iii the early stage of systole, before the 

 blood has left the ventricles, may be studied by applying a 

 ligature round the great vessels, and then plunging the heart 

 in a hot solution to cause it to contract round the contained 

 blood which cannot escape. 



Measurements of hearts so fixed show that, at the 

 beginning of contraction, the antero-posterior diameter is 

 increased, while the lateral diameter is diminished. In 

 contracting, the lateral walls appear to be pulled towards the 

 septum — the increase in the antero-posterior diameter being 

 largely due to the blood in the right ventricle pressing on 

 and pushing forward the thin wall of the infundibulum bdow 

 the pulmonary artery. 



As the ventricles drive out their blood, both antero- 

 posterior and lateral diameters are diminished — but the 

 diminution in the lateral direction is the more marked, and 

 thus the section of the heart tends to become more circular. 



