762 



THE THORAX. 



cle; it is longer, narrower, and more curved than that of the right side, and its 

 margins more deeply indented, presenting a kind of foliated appearance. Its 

 direction is forwards and towards the right side, overlapping the root of the 

 pulmonary artery. 



In order to examine its interior, a horizontal incision should be made along the attached 

 border of the auricle to the ventricle ; and from the middle -of this, a second incision should be 

 carried upwards. 



The following parts then present themselves for examination : 



The openings of the four pulmonary veins. 

 Auriculo-ventricular opening. 

 Musculi pectinati. 



The pulmonary veins, four in number, open, two into the right, and two into 

 the left side of the auricle. The two left veins frequently terminate by a com- 

 mon opening. They are not provided with valves. 



Fig. 420. The Left Auricle and Ventricle laid open, the Anterior Walls of both being removed. 



passed theAorUc opening 



The auricula-ventricular opening is the large oval aperture of communication 

 between the auricle and ventricle. It is rather smaller than the corresponding 

 opening on the opposite side. 



The musculi peciinati are fewer in number and smaller than on the right side; 

 they are conlinrd to the inner surface of the appendix. 



On the inner surface of the septum auriculurum may be seen a lunated impres- 

 sion, bounded below by a crescentic ridge, the concavity of which is turned 

 upwards. The depression is just above the fossa ovalis in the right auricle. 



The Left Ventricle is longer and more conical in shape than the right ven- 

 tricle. It forms a small pfart of the left side of the anterior surface of the heart, 

 and a considerable part of its posterior surface. It also forms the apex of the 

 lu-art by its projection beyond the right ventricle. Its walls are much thicker 

 than those of the right ventricle, the proportion being at least 3 to 1. They 



