34 TOPOGRAPHICAL ANATOMY OF THE 



differs somewhat from the corresponding cavity on the right side of the 

 heart. The auricle, the tip of which is slightly blunter than that of 

 the right auricle, is more or less dentated along its border, points in a 

 cranial direction, and fits into an angle formed by the pulmonary artery 

 and the base of the left ventricle. In the interior of the cavity the 

 wall is smooth, except in the auricle, in which part alone there are 

 musculi pectinati. The number of openings from the pulmonary veins 

 into the atrium is variable (from four or five to eight or nine). The 

 atrio-ventricular orifice (ostium ^ atrioventriculare sinistrum) is oval in 

 outline and somewhat smaller than the corresponding opening in the 

 right atrium, though the size of the opening depends largely on the 



Fig. 15. — Key outline of the heart to show the position of the 

 sections illustrated in Figs. 16 and 17. 



condition of contraction or relaxation of the muscular wall of the 



ventricle. 



Dissection. — To open the left ventricle an incision should begin near 

 the coronary groove and be carried parallel to one longitudinal groove 

 round the apex and up the other side parallel to the other longitudinal 

 groove. 



Ventriculiis sinister. — The left ventricle differs from the right in 



its general shape. It is conical in conformity with the form of the 



ventricular mass of the heart, and its apex forms the apex of the heart 



as a whole. Owing to the thickness of its wall, combined with the 



fact that the interventricular septum is concave towards the left, a 



transverse section of the left ventricle would present an oval or rounded 



outline. At the same time it must be noted that the wall of the 



cavity is not of uniform thickness. It is thin at the apex, and the 



septum is divisible into a lower thick muscular part (septum musculare) 



and a thinner upper part (septum membranaceum). 



1 Ostium [L.], (os, a mouth ; in a borrowed sense, an entrance), a door or entrance. 



