THORACIC CAVITY 107 



The Left Atrio - ventricular Orifice. The left atrio- 

 ventricular orifice lies in the lower and posterior part of the 

 ventricle, posterior to the left margin of the sternum, at the 

 level of the fourth left costal cartilage. It is somewhat smaller 

 than the right atrio-ventricular orifice and admits the tips of 

 two fingers only, a fact which will be better appreciated when 

 the orifice is examined from the left atrium, at a later period. 

 It is guarded by a valve, formed by two cusps and called, 

 therefore, the bicuspid valve^ which prevents regurgitation of 

 blood from the left ventricle into the left atrium. 



Valvula Bicuspidalis. The bicuspid, mitral or left atrio- 

 ventricular valve consists of two cusps, a large superior 

 (anterior) and a small inferior (posterior). Occasionally, 

 however, as on the right side, small additional cusps are 

 interposed between the bases of the main cusps. The bases 

 of the cusps are attached to a fibrous ring which surrounds 

 the atrio-ventricular orifice, and their apices project into the 

 cavity of the ventricle. To their apices, margins and ventri- 

 cular surfaces are attached the chordae tendineae from the 

 papillary muscles, which hold the margins of the cusps 

 together and prevent the valve being driven backwards into 

 the atrium during the contraction of the ventricle. The 

 dissector should note, however, that the chordae tendineae 

 spread less over the ventricular surface of the superior cusp 

 than over that of the inferior cusp, and he should associate 

 this fact with the circumstance that blood flows over both 

 surfaces of the large superior cusp, which intervenes between 

 the atrio-ventricular and the aortic orifices. By means of the 

 large superior cusp of the mitral valve the cavity of the 

 ventricle, which has, on the whole, a somewhat conical form, 

 is converted into a bent U-shaped tube, one limb of the tube 

 lying below and to the left, and the other anteriorly and to 

 the right. The blood enters the ventricle below and 

 posteriorly through the atrio-ventricular orifice. It runs 

 forwards towards the apex of the cavity along the inferior 

 surface of the superior cusp of the mitral valve, then, as 

 the ventricle contracts, it is driven upwards, backwards, 

 and to the right, to the aortic orifice, along the anterior 

 surface of the large anterior cusp of the mitral valve. 

 The portion of the cavity of the left ventricle which lies 

 directly below the aortic orifice is known as the aortic 

 vestibule (Fig. 53). Its walls consist mainly of fibrous tissue ; 



