HEART LEFT VENTRICLE. 763 



are also thickest in the broadest part of the ventricle, becoming gradually thin- 

 ner towards the base, and also towards the apex, which is the thinnest part. 



Its cavity should be opened, by making an incision through its anterior wall along the left 

 side of the ventricular septum, and carrying it round the apex and along its posterior surface to 

 the auriculo-ventricular opening. 



The following parts present themselves for examination : 



^ . ( Auriculo-ventricular. -* r i ( Mitral. 



Openings, ^^ Yalves. g emilunar , 



Chordae teudineas. Columnse carneae. 



The auricula-ventricular opening is placed to the left of the aortic orifice, 

 beneath the right auriculo-ventricular opening, opposite the centre of the ster- 

 num. It is a little smaller than the corresponding aperture of the opposite side; 

 and, like it, is broader in the transverse than in the antero-posterior diameter. 

 It is surrounded by a dense fibrous ring, covered by the lining membrane of 

 the heart, and guarded by the mitral valve. 



The aortic opening is a small circular aperture, in front and to the right side 

 of the auriculo-ventricular, from, which it is separated by one of the segments 

 of the mitral valve. Its orifice is guarded by the semilunar valves. Its position 

 corresponds to the sternum, on a line with the lower border of the third left 

 costal cartilage. 



The mitral valve is attached to the circumference of the auriculo-ventricular 

 orifice, in the same way that the tricuspid valve is on the opposite side. It is 

 formed by a duplicature of the lining membrane, strengthened by fibrous tissue, 

 and contains a few muscular fibres. It is larger in size, thicker, and altogether 

 stronger than the tricuspid, and consists of two segments of unequal size. The 

 larger segment is placed in front, between the auriculo-ventricular and aortic 

 orifices, and is said to prevent the filling of the aorta during the distension of 

 the ventricle. Two smaller segments are usually found at the angle of junction 

 of the larger. The mitral valve flaps are furnished with chords tendineae ; the 

 mode of attachment of which is precisely similar to those on the right side ; 

 but they are thicker, stronger, and less numerous. 



The semilunar valves surround the orifice of the aorta; they are similar in 

 structure, and in their mode of attachment, to those of the pulmonary artery. 

 They are, however, larger, thicker, and stronger than those of the right side; 

 the lunulae are more distinct, and the corpora Arantii larger and more promi- 

 nent. Between each valve and the cylinder of the aorta is a deep depression, 

 the sinus aortici (sinuses of Valsalva) ; they are larger than those at the root of 

 the pulmonary artery. 



The columns? earnest admit of a subdivision into three sets, like those upon 

 the right side ; but they are smaller, more numerous, and present a dense inter- 

 lacement, especially at the apex, and upon the posterior wall. Those attached 

 by one extremity only, the musculi papillares, are two in number, being con- 

 nected one to the anterior, the other to the posterior wall ; they are of large 

 size, and terminate by free rounded extremities, from which the chordae tendineas 

 arise. 



The Endocardium is the serous membrane which lines the internal surface of 

 the heart ; it assists in forming the valves by its reduplications, and is continu- 

 ous with the lining membrane of th.e great bloodvessels. It is a thin, smooth, 

 transparent membrane, giving to the inner surface of the heart its glistening 

 appearance. It is more opaque on the left than on the right side of the heart, 

 thicker in the auricles than in the ventricles, and thickest in the left auricle. It 

 is thin on the musculi pectinati, and on the columnaa carneae; but thicker on 

 the smooth part of the auricular and ventricular walls, and on the tips of the 

 musculi papillares. 



Structure of the Heart. The heart consists of muscular fibres, and of fibrous 

 rings which serve for their attachment. 



