160 Mammalia. 



indication of the superficial semi-lunar fossa (whicli is well 

 marked in man), may be seen ; i.e. the now adherent border 

 of a membranous valve which during foetal life is applied to 

 the left side of the then open Foramen Ovale. The mouths 

 of each pair of Pulmonary veins open close to one another ; 

 those from the right lung open into the extreme right of the 

 Auricle, near the septum ; those from the left lung open in 

 both cases close to the left Yena Azygos. The Pulmonary 

 veins are not provided with valves. 



Dissection of Left Ventricle. In order to expose the 

 internal aspect of the left Ventricle, an incision should be 

 made through its anterior wall, just below the Coronary 

 artery that supplies the left side of the Heart, and to the left 

 of the septum ; and should be continued parallel with the ven- 

 tricular groove round the right of the Apex to a point a little 

 to the left of the middle line posteriorly, and then upwards 

 until it; meets the left Coronary artery at the point where it 

 descends to run along the ventricular groove at the back of 

 the Heart : on raising this triangular flap it will be necessary 

 to cut across several small fibrous bands which run from the 

 free outer wall of the Ventricle to the Inter-ventricular 

 Septum. 



The walls of this Ventricle will at once be noticed to be 

 considerably thicker than those of the right Ventricle. Its inter 

 nal surface is much marked by the projecting Columnae Car- 

 neae, except near the Aortic opening. The Musculi Papillares 

 are collected for the most part into two great bundles, whence 

 proceed the numerous Chordae Tendineae with which the two 

 segments of the Auriculo-ventricular valve are furnished. 

 This valve, which is called * Mitral' from a fancied resemblance 

 to a Mitre, is attached both to a fibrous ring surrounding the 

 Auriculo-ventricular aperture, and, in part, to the Aortic ring : 

 the right segment of this valve alone intervening between the 

 the Aortic and Auriculo-ventricular apertures. The open- 



