THE HEART. 



595 



The unitive fibres constitute two thin bands — a right and left, caiTied from 

 one auricle to the other. 



The proper fibres are divided into several fasciculi, some of which are arranged 

 in rings around the auriculo- ventricular opening ; others in interwoven loops ; 

 and others, again, in sphincters, which suiTound the entrance of the veins. 



These fibres are arranged in such a manner that, in contracting, they diminish 

 the auricles by their superior and lateral planes and extremities, and propel the 

 blood towards the auriculo-ventricular openings. 



(The arrangement of the muscular fibres constitutes the most remarkable 



Fig. 357. 



POSTERIOR UNITIVE FIBRES, AND FIBRES OF THE POSTERIOR FACE OF THE AURICLES, 



1, Posterior unitive fibres; 2, fibres of the right auricle; 3, fibres of the posterior vena cava. 



feature in the anatomy of the heart. We have seen that the auricles, as well as 

 the ventricles, possess not only fibres proper to each compartment, but also 

 unitive or common fibres, which assure the simultaneousness in action of the 

 similar or homologous cavities. Besides, the fibres of the auricles and those of 

 the ventricles are distinct, and not continuous ; so that, from their independence 

 of each other, it results that these two sections of the heart may act separately, 

 and contract, not simultaneously, but alternately — a condition indispensable to 



