250 



THE HEAET. 



near the apex, are three times as thick as those of the right ventricle, 

 are tliickest at the part where the ventricle is widest, about one-fourth 

 of its length from the base (fig. 170, 3, 3'); from this point they 

 become thinner towards the auricular opening, and still thinner towards 

 the apex (3"), which is, therefore, the weakest part. The lining mem- 

 brane, 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 columnte earner, 

 musculi papillares with chordie tendineaj, and two orifices guarded with 

 valves. The columiue earner are smaller than those of the right ven- 

 tricle, 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 u])per part of the anterior wall and 

 septum is comparatively smooth. The musnill papilJares {i, 5) are col- 

 lected into two groups, which are larger than those of the right ven- 

 tricle. The two orifices of this ventricle are situated very close together, 

 with one of the segments (fig. 171, 6) of the auriculo-ventricular valve 



Fi<r. 171. 



Fig. 171. — View of the Base of the Ventricular Part of the Heart, showing 



THE relative POSITION OP THE ArTERIAL AND AuRICULO-VENIKICULAR ORIFICES 



(Allen Thomson). Two-thirds the Natural Size. 



The muscular fibres of the ventricles are exposed by the removal of the pericardium, 

 fat, bloodvessels, &c. ; the pulmonary artery and aorta and the auricles have been re- 

 moved. The valves are in the closed condition. A, is placed opposite that part which 

 is most anterior when the oi'gan is in the natural ijosition within the body. 1, 1, right 

 ventricle ; 1', conus arteriosus ; 2, 2, left ventricle ; 3, 3, the divided wall of the right 

 auricle ; 4, that of the left ; 5, the anterior, 5', the right (or inferior), and 5", the 

 septal segment of the tricuspid valve ; 6, the anterior or aortic, and 6', the posterior 

 or parietal segment of the mitral valve. In the angles between these segments are seen 

 smaller fringes ; 7, the pulmonary artery ; 8, placed upon the root of the aorta ; 9, the 

 posterior, 9', the anterior coronary artery. 



between : the auricular opening is placed at the left and posterior part 

 of the base of the ventricle ; the aortic opening, in close proximity in 

 front and towards the right. 



The hicuspid or mitral valve, at the left auricular opening, resembles 

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



