HEART LEFT VENTRICLE. 697 



'he auricula-ventricular opening is placed to the left of the aortic orifice, be- 

 neath the right auriculo- ventricular opening, opposite the centre of the sternum. 

 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 is 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 corre- 

 sponds 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 ori- 

 fices, 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 valves are furnished with chords tendineas ; their mode of 

 attachment 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 

 lunulas are more distinct, and the corpora Arantii larger and more prominent. 

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

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

 pulmonary artery. 



The columnae carneae admit of a subdivision into three sets, like those upon 

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

 ment, especially at the apex, and upon the posterior wall. Those attached by one 

 extremity only, the musculi papillares, are two in number, being connected 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, by its reduplications, the valves contained in this 

 organ, and is continuous with the lining membrane of the 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 columns carneas ; 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. 



The fibrous rings surround the auriculo-ventricular and arterial orifices ; they 

 are stronger upon the left than on the right side of the heart. The auriculo- 

 ventricular rings serve for the attachment of the muscular fibres of the auricles 

 and ventricles, and also for the mitral and tricuspid valves ; the left one is closely 

 connected, by its right margin, with the aortic arterial ring. Between these and 

 the right auriculo-ventricular ring, is a fibre-cartilaginous mass ; and in some of 

 the larger animals, as the ox and elephant, a portion of bone. 



The fibrous rings surrounding the arterial orifices serve for the attachment of 

 the great vessels and semilunar valves. Each ring receives, by its ventricular 

 margin, the attachment of the muscular fibres of the ventricles; its opposite 



