466 



THE BLOOD-VASCULAR SYSTEM. 



its posterior surface. It also forms the apex of the heart by its projection beyond 

 the right ventricle. Its walls are much thicker than those of the right side, the 

 proportion being as 3 to 1. They are thickest opposite the widest part of the 

 ventricle, becoming gradually thinner toward the base, and also toward the apex, 

 which is the thinnest part. 



The following parts present themselves for examination : 



f Auriculo-ventricular. vi,r,c, / Mitral 



Openings j KQ ^ Valves j Semilunar> 



Chordae tendinese. Columnae carneae. 



The auricula-ventricular opening is placed below and to the left of the aortic 

 orifice. It is a little smaller than the corresponding aperture of the opposite side, 



Bristle passed through left 

 auricula-ventricular opening. 



'assed through aortic opening. 



FIG. 277. The left auricle and ventricle laid open, the posterior walls of both being removed. 



admitting only two fingers ; but, 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 valves. 



The aortic opening is a 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. The portion of the 

 ventricle immediately below the aortic orifice is often termed the aortic vestibule 

 of Sibson. It possesses fibrous instead of muscular walls, and so does not collapse 

 during the ventricular diastole ; it thus gives space for the segments of the aortic 

 valve during its closure. 



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 and to the right between the auriculo-ventricular 



