232 PHYSIOLOGY OF THE HEART. [CH. xx. 



diastole. These different parts of the cycle must next be studied 

 in detail. 



The Auricular Diastole. During this time, the blood from the 

 large veins is flowing into the auricles, the pressure in the veins 

 though very low being greater than that in the empty auricles. 

 The blood expands the auricles, and during the last part of 

 the auricular 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 j unction 

 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 ven- 

 tricles is chiefly brought about in virtue of their elasticity ; this 

 is particularly evident in the left ventricle with its thick muscular 

 coat. It is equal to 23 mm. of mercuiy, 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 

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



