564 THE VASCULAR SYSTEMS 



fingers; but, like it, is broader in the transverse than in the antero-posterior diameter. 

 Its right, posterior, and left sides are surrounded by a dense horseshoe-shaped, 

 fibrous ring (annulus fibrosus sinister). The orifice is guarded by the mitral 

 or bicuspid valve. 



The aortic opening (ostium arteriosum) is a circular aperture, in front and to 

 the right side of the auriculoventricular opening, from which it is separated by 

 the aortic cusp of the mitral valve. Its orifice is guarded by the aortic valve, which 

 consists of three semilunar segments. The portion of the ventricle immediately 

 below the aortic orifice is often termed the aortic vestibule, and possesses fibrous 

 instead of muscular walls. 



fistle passed through left 

 auriculoventricular opening. 



Passed through aortic opening. 



i 

 FIG. 419. The left auricle and ventricle laid open, the posterior walls of both being removed. 



The mitral or bicuspid valve (valvula bicuspidalis) is attached to the circumfer- 

 ence of the auriculoventricular orifice in the same way that the tricuspid valve is 

 on the opposite side. It consists of two triangular cusps, formed by duplicatures 

 of the lining membrane, strengthened by fibrous tissue, and containing a few 

 muscle fibres. The cusps are of unequal size, and are larger, thicker, and stronger 

 than those of the tricuspid valve. The larger segment, the anterior or aortic cusp 

 (cuspis anterior), is placed in front and to the right between the auriculoventricular 

 and aortic orifices; the smaller, the posterior or marginal cusp (cuspis posterior), is 

 placed to the left and behind the opening. Two smaller cusps are usually found at 

 the angles of junction of the larger. The cusps of the mitral valve are furnished 

 with chordae tendineae, which are attached in a manner similar to those on the 

 right side; they are, however, thicker, stronger, and less numerous. 



The aortic valve consists of three semilunar segments (valvulae semilwiares 

 aortae), which surround the orifice of the aorta; two are posterior (right and left) 

 and one anterior. They are similar in structure and in their mode of attachment 

 to those of the pulmonary valve, but are larger, thicker, and stronger; the lunulse 

 are more distinct and the corpora Arantii thicker and more prominent. Opposite 



