696 



HUMAN ANATOMY. 



directed forward, and which represents the free edge of the posterior segment or fold 

 of the auricular septum (page 708). In the floor is situated the large circular 

 aiiriculo-ventricular orifice by which the cavity of the auricle communicates with that 

 of the left ventricle. 



The Ventricles. — The two ventricles present many features in common and 

 may be described together, such differences as exist between them being pointed out 

 as the description proceeds. Each has a form which may be likened to a three-sided 

 pyramid whose base is directed upward and the apex downward. The edges of the 

 left ventricle are, however, somewhat more rounded than those of the right, so that 

 its form approaches more nearly that of a cone ; and, furthermore, it is somewhat 



Fig. 659. 



Systemic aorta 



Left pulmonary artery 



Superior left pulmonary vein 



Left auricular 



appendix 



Part of posterior 

 leaflet of mitral 

 valve 



Anterior (aortic) 

 leaflet of mitral 

 valve 



Left ventricle 



Superior vena cava 

 Azygos vein 



Right pulmonary 

 artery 



Superior right 

 pulmonarj' vein 



Inferior right 

 pulmonary vein 



Left auricle, 

 opened 



Inferior vena cava 



Coronary sinus, cut 



Interior of lell auricle and ventricle, seen from behind ; posterior wall of heart 

 has been partially removed by frontal section. 



longer than the right, its apex alone forming the apex of the heart. The surfaces 

 presented by each ventricle may be termed antero-lateral, posterior, and median, but 

 in using these terms the heart is to be regarded as placed so that its long axis is ver- 

 tical ; in sihi the antero-lateral surfaces look largely upward and the posterior sur- 

 faces downward. The median wall is a partition, the interveiitricidar scptjivi (septum 

 ventriculorum), common to the two ventricles, and completely separates their cavities. 

 Throughout the greater part of its extent this septum is muscular, but towards its 

 upper border it becomes fibrous {pars metnbranacea) and is continuous with the 

 septum of the auricles ; the position of its edges is indicated upon the external sur- 

 face of the heart by the anterior and posterior interventricular grooves. The bases of 



