8o THORAX 



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 bicuspid valve, called the mitral valve, 

 which prevents regurgitation of blood from the left ventricle 

 into the left atrium. 



The Mitral Valve. The mitral or left atrio-ventricular 

 valve consists of two cusps, a large anterior and a small 

 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 ventricular 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 

 anterior than over that of the posterior cusp, and he should 

 associate this fact with the circumstance that blood flows over 

 both surfaces of the large anterior cusp, which intervenes between 

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

 this large anterior 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 anteriorly to- 

 wards the apex of the cavity along the inferior surface of 

 the anterior cusp of the mitral valve, then, as the ventricle 

 contracts, it is driven upwards, anteriorly, 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. 40). Its walls con- 

 sist mainly of fibrous tissue ; therefore they remain quiescent 

 during the contraction of the ventricle and, as a result, the 

 rapid closure of the aortic valve is not interfered with when 



