THE CIRCULATION. 233 



tricle. If the same quantity of blood is not ejected by the two 

 ventricles at each contraction the blood accumulates somewhere in 

 the vascular system. 



Whilst the blood-pressure in the lungs is very low, the varia- 

 tions in pressure are very small, not more than 10 to 15 millimeters 

 in physiological conditions. The variations of blood-pressure in the 

 systemic circulation are much greater. Hence the work of the right 

 ventricle does not vary much,, whilst that of the left ventricle must 

 vary considerably; hence it is more easily fatigued than the right 

 ventricle. 



If the left ventricle is not able to eject all its blood on account 

 of the resistance in the periphery, then the blood cannot flow- 

 readily from the right heart. As a rule, this effect on the right 

 heart by an incomplete evacuation of the left ventricle is counter- 

 acted by the great capacity of the pulmonary vessels, which 

 accommodate this backing up of blood and permits it to absorb the 

 greatest quantity of oxygen. The resistance to the flow of blood in the 

 pulmonaiy circulation is very small, and the blood makes its circuit in 

 6 to 7 seconds. 



GENERAL SHAPE OF THE HEART. 



During ventricular systole the heart undergoes a torsion 

 around its vertical axis; a twisting from left to right anteriorly by 

 which the left ventricle gets in front. The ventricles during their 

 diastole have a general conical form which during their systole 

 becomes globe-like. This change of form also alters the diameter 

 of the heart. The vertical diameter is lessened by the contraction 

 of the cardiac muscle. The transverse diameter from right to left 

 is diminished by the change to a sphere during the systole of the 

 ventricles. There is an increase in the antero-posterior diameter 

 from the spherical form of the ventricle. The heart is in close 

 contact with the walls of the chest by this globular form during the 

 systole. 



CHANGE IN VOLUME. 



In plethysmographic studies of the heart it is found that there 

 is a diminution of its volume during ventricular systole. 



CHANGE IN CONSISTENCY. 



The hardening of the heart during systole is of great impor- 

 tance in the explanation of the cardiac impulse. If you simply 

 touch the heart of a living animal you obtain considerable informa- 

 tion as to the energy of the cardiac contraction. 



