CIRCULATION 



237 



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 may be investigated after death 

 stiffening has passed off and has left the walls relaxed. 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 in the early stage of si/stole, 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 begin- 



AWICLES 



VfffTKtCLfS 



FIG. 113. Scheme of the Cardiac Cycle in the Human Heart. A.S , auricular 

 systole : V.S. , ventricular systole ; P., pause. 



ning of contraction the an tero- 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 oil and pushing 

 forward the thin wall of the conus. 



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. 



There is no great shortening in the long axis of the heart. 

 Although the contraction of the longitudinal fibres tends to 

 approximate base and apex, this is in part prevented by the 

 contraction of the circular fibres. 



5. Changes in the Position of the Heart. During contraction 

 the heart undergoes, or attempts to undergo, a change in 



