138 THE BLOOD AND ITS CIRCULATION. 



tion of the auricle (one-fifth) ; the three following fifths, by 

 the contraction of the ventricle (three-fifths) ; in the last 

 fifth, the heart being in entire repose (see the table, p. 141). 

 We know that during these four latter fifths (three-fifths of 

 ventricular systole and one-fifth of total repose) the auricle 

 is quite still. Speaking generally, the revolution of the heart 

 is divided into three periods : the first, of auricular systole ; 

 the second, of ventricular systole; the last, of entire repose. 

 The typical length which we have assigned to these three 

 periods may vary greatly, according to circumstances and 

 individuals, and even in animals : the second period, that of 

 repose, presents the greatest number of varieties : among the 

 cold-blooded animals, the batrachians particularly, there is a 

 long interval of repose after each contraction of the heart. 



But why, when the heart is in repose, does not the blood 

 which has been driven into the artery return to the ventricu- 

 lar cavity ? Because the arterial orifice (pulmonary or aortic) 

 is furnished with three semilunar or sigmoidal valves,- which 

 are thrown out by the retrograde pressure of the blood, and 

 completely close the corresponding orifice. There is no need 

 of a lengthened explanation of this mechanism,- which is 

 plain to any one who will dissect a heart. At the moment 

 when the blood has a tendency to flow back again, the fob- 

 like form of these valves, the orifice of which is turned 

 towards the arterial cavity, presents a sort of trap to the 

 blood, by which the valves are forced out, and thus occlude 

 the passage. The nodule of Arentius, which is placed in 

 the middle of the free edge of each of these valves, has, no 

 doubt, the effect of making the occlusion more complete. 



To sum up : 



1. The auricle contracts instantaneously and without much 

 force, that it may throw the blood into the ventricle, which is 

 only too ready to receive it. At all other times the auricle is in 

 a state of relaxation, of slow and progressive distention, which 

 produces the effect of blood-letting at the terminal extremity 

 of the venous system. 



2. The ventricle contracts strongly and slowly, on account 

 of the resistance which it has to overcome, and which is 

 occasioned by the tension of the blood by previous contrac- 

 tions accumulated in the arteries. 



The auriculo-ventricular valves are not valves, but an en- 

 tirely distinct apparatus. 



The semilunar valves are true valves. 



Sounds and Impulse of the Heart. Hitherto we have 



