CH. XXI.] THE CARDIAC CYCLE 233 



diastole it passes on into the ventricles. The dilatation of the 

 auricles is assisted by the elastic traction of the lungs. The lungs 

 being in a closed cavity, the thorax, and being distended with air, 

 are in virtue of their elasticity always tending to recoil and squeeze 

 the air out of their interior ; in so doing they drag upon any other 

 organ with which their surface is in contact: this elastic traction 

 will be greatest when the lungs are most distended, that is during 

 inspiration, and will be more felt by the thin-walled auricles than by 

 the thick -walled ventricles of the heart. 



The Auricular Systole is sudden and very rapid ; by contracting, 

 the auricles empty themselves into the ventricles. The contraction 

 commences at the entrance of the great veins, and is thence pro- 

 pagated towards the auriculo-ventricular opening. The reason why 

 the blood does not pass backwards into the veins, but onward into 

 the ventricles, is again a question of pressure ; the pressure in the 

 relaxed ventricles, which is so small as to exert a suction action on 

 the auricular blood, is less than in the veins. Moreover, the 

 auriculo-ventricular orifice is large and widely dilated, whereas the 

 mouths of the veins are constricted by the contraction of their 

 muscular coats. Though there is no regurgitation of the blood 

 backwards into the veins, there is a stagnation of the flow of blood 

 onwards to the auricles. The veins have no valves at their entrance 

 into the auricles, except the coronary vein, which does possess a 

 valve ; there are valves, however, at the junction of the subclavian 

 and internal jugular veins. 



Ventricular Diastole ; during the last part of the auricular diastole 

 and the whole of the auricular systole, the ventricles have been 

 relaxed and then filled with blood. The dilatation of the ventricles 

 is chiefly brought about in virtue of their elasticity ; this is particu- 

 larly evident in the left ventricle, with its thick muscular coat. It 

 is equal to 23 mm. of mercury, and is quite independent of the 

 elastic traction of the lungs, which, however, in the case of the 

 thinner-walled right ventricle comes into play. 



The Ventricular Systole ; this is the contraction of the ventricles, 

 and it occupies more time than the auricular systole; when it 

 occurs the auriculo-ventricular valves are closed and prevent re- 

 gurgitation into the auricles, and when the force of the systole 

 is greatest, and the pressure within the ventricles exceeds that in the 

 large arteries which originate from them, the semilunar valves are 

 opened, and the ventricles empty themselves, the left into the aorta, 

 the right into the pulmonary artery. Each ventricle ejects about 

 3 ounces of blood with each contraction ; the left in virtue of its 

 thicker walls acts much more forcibly than the right. The greater 

 force of the left ventricle is necessary, as it has to overcome the 

 resistance of the small vessels all over the body ; whereas the right 



