THORACIC CAVITY 79 



wall of the left ventricle forwards till the base of a large papillary muscle 

 which springs from its internal surface is exposed ; carry the incision 

 anterior to this and then onwards to the apex, and remove the anterior 

 wall of the left ventricle and the anterior part of the inter-ventricular 

 septum. The cavity of the left ventricle and the mitral valve, which guards 

 the left atrio-ventricular orifice, are now exposed (Fig. 40). 



Ventriculus Sinister. The cavity of the left ventricle is 

 longer and narrower than that of the right ventricle. It 

 reaches to the apex, and when exposed from the front it 

 appears to be of conical shape. In cross section it has a 

 circular or broadly oval outline, and its walls are very much 

 thicker than those of the right ventricle (Fig. 37). When 

 the interior has been cleaned with the aid of a sponge and 

 forceps, the dissector will note that its walls are covered with 

 a dense mesh-work of trabeculae carneae, which are finer but 

 much more numerous than those met with in the right 

 ventricle. The network is especially complicated at the 

 apex and on the inferior wall of the ventricle, whilst the 

 surface of the septum and the upper part of the anterior wall 

 are, comparatively speaking, smooth. But whilst the trabeculae 

 carneae in the left ventricle are slighter and more numerous 

 than those in the right, the musculi papillares, on the other 

 hand, are less numerous and much stronger; indeed, as a 

 general rule there are only two papillary muscles in the left 

 ventricle, an anterior and an inferior, the former attached to 

 the anterior wall and the latter to the inferior wall of the 

 cavity. The chordae tendineae from the papillary muscles 

 pass to the margins and to the ventricular surfaces of the two 

 cusps of the mitral valve, which guards the left atrio-ventricular 

 orifice, the chordae tendineae from each papillary muscle 

 gaining attachment to the adjacent margins of both cusps. 



Dissection. Detach the anterior papillary muscle from the anterior 

 wall of the ventricle and note that its chordae tendinese go to the anterior 

 and left margins of the cusps of the mitral valve. Introduce the blade of 

 a scalpel between the anterior margins of the cusps and carry it downwards 

 between the groups of chordse going to the apex of the papillary muscle ; 

 then split the papillary muscle from its apex to its base leaving each half 

 connected with a corresponding group of chordae tendinese. The cusps of 

 the mitral can now be separated from each other, and the atrio-ventricular 

 orifice and the cavity of the ventricle can be more completely examined. 



The Orifices of the Left Ventricle. There are two orifices 

 of the left ventricle one of entrance, the left atrio-ventricular 

 orifice, and one of exit, the aortic orifice. 



The Left Atrio - ventricular Orifice. The left atrio- 



