44 



THE HEART 



The right auricle of the human heart, therefore, consists of 

 musculature from three sources (1) Sinus venosus. (2) Auricular 

 canal. (3) Auricle proper. 



The left auricle is composed of musculature from (1) The 

 auricular canal. (2) The auricle proper. (3) Possibly sinus 

 venosus. All these parts are in the freest muscular continuity. 



The Ventricles of the Human Heart. The main mass of these 

 is true ventricle, although, as we have shown above, the invaginated 



FIG. 5. The heart from behind, showing the arrangement of the musculature 

 of the left auricle and ventricle (Keith) A', the auricular base of the left ventricle 

 in systole of the auricle ; A, its position in ventricular systole. 



portion of the auricular canal (4.4, Fig. 1) is represented by the 

 A-V bundle. The single ventricle of the primitive heart is a 

 diverticulum from the ventral wall of the primitive cardiac tube. 

 The double chamber of the mammalian heart is homologous with 

 it. The question of how the septum arose has, however, been 

 differently answered. The old idea was that the interventricular 

 septum grew up from the apex and thus divided the common 

 cavity. This view is undoubtedly incorrect. What really happens 

 is that the two ventricles are developed side by side from the 



