222 HUMAN PHYSIOLOGY 



3. Changes in Shape of the Chambers. 



1. Auricles. These simply become smaller in all direc- 

 tions 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 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 it while it is still beating and plunging it in some 

 hot solution to fix its contraction. 



The condition at the beginning 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 begin- 

 ning 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 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. 



4. Change in Position of the Heart. During contraction 

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

 position. In the relaxed condition it hangs downwards and 

 to the left from its plane of attachment, but when it becomes 

 rigid in ventricular contraction it tends to take a position at 

 right angles to its base Cor sese erigere, as Harvey describes 

 the movement. Since the apex and front wall are in contact 

 with the chest, the result of this movement is to press the 



