THE HEART. 



621 



called ostium arteriosum, or the left auriculo-ventricular 

 opening. 



Left ventricle. This cavity presents a conical form, and is 

 posterior to the right ventricle. Its "base is above, and apex, 

 which is below, projects be- 

 yond the right, and consti- 

 tutes the apex of the heart. 

 Its walls are about three 

 times as thick as those of 

 the right ventricle, and it 

 has the same arrangement 

 of columnce carnece and chor- 

 dce tendinece, as the right, 

 only thicker and stronger. 



The partition between 

 the two ventricles is styled 

 the septum cordis. It con- 

 sists of a thick muscular 

 wall, formed mostly by the 

 left ventricle. At the upper and back part of this cavity 

 the left auriculo-ventricular opening is seen, and is found to 

 be protected by a fold of the lining membrane projecting 

 from round the margin of this orifice into the ventricle, and 

 dividing into two portions, called the mitral valve. The 

 margin of this opening consists, as in the right, of a white 

 and dense tendinous structure. 



The anterior division of the mitral valve is larger and 

 broader than the posterior, and covers, in a great measure, 

 the aortic opening. This valve, as well as the tricuspid, 

 contains the fibrous structure, is attached by tendinous 

 chords to the columna? earner, and serves to prevent the 

 reflux of blood into the auricle. 



The orifice of the aorta is seen in front of the auricular 

 opening, #nd, like that of the pulmonary artery, is guarded 



FIG. 200 represents the left Ventricle laid open, a Parietes of left ven- 

 tricle. 6 Its cavity, c Mitral valve, d Chorda? tendineae. e Columnse 

 carneae. / Right auricle, g Left auricle, h h Four pulmonary veins, i 

 Aorta . j Pulmonary vein. 



