362 



THE HEART. 



which, excepting near the apex, is nearly three times as thick as that of the 

 right ventricle, is thickest at the part where the ventricle is widest, about one- 

 fourth of its length from the base (fig. 312, 3) ; from this point it becomes thinner 

 towards the auricular opening, and still thinner towards the apex (3"), which is, 

 therefore, the weakest part. The lining membrane, which is continuous with that 

 of the left auricle and the aorta, is usually less transparent than that of the right 

 ventricle, especially in later life. In the interior of the cavity are noticed columnse 

 carneae, musculi papillares with chordae tendineae, and two orifices guarded by valves. 

 The columns earner are smaller than those of the right ventricle, but are more 

 numerous and more closely reticulated. Their intersections are very numerous near 

 the apex of the cavity, and also along its posterior wall, but the upper part of the 

 anterior wall and septum is comparatively smooth. The musculi papillares (4, 5) 

 are collected into two groups, which are larger than those of the right ventricle. 

 The two orifices of this ventricle are situated very close together, with one of the 

 segments (fig. 313, 6) of the auriculo-ventricular valve between : the auricular 

 opening is placed at the lower and posterior part of the base of the ventricle ; the 

 aortic opening, in close proximity, in front and higher. 



The bicuspid or mitral valve (fig. 312, 6, 6'), at the left auricular opening, 

 resembles in structure the tricuspid valve of the right ventricle, but it is much 



Fig. 314. THE SEMILUNAR VALVES OP THE AORTA AND PULMONARY ARTERY, SEEN FROM THEIR DISTAL 



SIDE. (Allen Thomson.) 



I, transverse section of the pulmonary artery immediately above the attachment of the semilunar 

 valves : a, the left, and c, the right anterior segments ; b, the posterior segment : opposite each the 

 sinus of Valsalva is seen, and the attachment of the valve-segment to the inner wall of the artery. 



II, a similar section of the aorta : a, the left posterior segment, and b, the anterior segment, with 

 the corresponding sinuses of Valsalva, from which the coronary arteries are seen to take their origin ; c, 

 the right posterior segment ; d, the right, and e, the left coronary arteries. 



thicker and stronger in all its parts, and consists of only two pointed segments, 

 continuous at .their attached bases. The larger of the two segments is suspended 

 obliquely to the right and in front of the other, between the auricular and 

 aortic openings ; the smaller to the left and behind, and close to the wall of the 

 ventricle. There is usually a smaller lobe at each angle of junction of the two 

 principal segments, more apparent than those between the segments of the tricuspid 

 valve. 



As on the rig-lit side, the two sets of chordae tendineaa from the papillary muscles proceed 

 each to an angle between the two segments, and are attached in like manner to their 

 margins and ventricular surfaces (fig 1 . 312, and fig. 315, B, e), so that the musculi papillares, 

 when they contract, tend to bring the edges of the flaps together. The chorda? tendineae 

 are stronger and less numerous than in the right ventricle. Small bands, partly muscular 

 partly tendinous, may often be found crossing the cavity of the ventricle in various direc- 

 tions. They are vestiges of the spongy structure of the cavity which is found during a part 

 of embryonic life. 



