THE CHAMBERS OF THE HEART. 877 



In the angle between the two limbs is a thick ledge of muscle, the supra- 

 ventricular crest. 



The right atrio-ventricular orifice is guarded by a tricuspid valve. The three 

 cusps are an anterior, which intervenes between the atrio-ventricular orifice 

 and the infundibulum, a medial or septal, and an inferior. Each cusp consists 

 of a fold of endocardium, strengthened by a little intermediate fibrous tissue. 

 The bases of the cusps are generally continuous with one another at the atrio- 

 ventricular orifice, where they are attached to a fibrous ring, but they may be 

 separated by small intermediate cusps which fill the angles between the main 

 segments. The apices of the cusps project into the ventricle. The margins, 

 which are thinner than the central portions, are notched and irregular. The 

 atrial surfaces are smooth. The ventricular surfaces are roughened, and, like the 

 margins and apices, they give attachment to fine tendinous cords, the chordae 

 tendinese. The opposite extremities of the chordae tendineae are attached to 

 muscular bundles, the musculi papillares, which project from the wall into the 

 cavity of the ventricle. 



The pulmonary orifice, which lies anterior and to the left of the tricuspid orifice, 

 is guarded by a pulmonary valve composed of three semilunar segments, two of 

 which are placed anteriorly and one posteriorly. The convexity or outer border 

 of each semilunar segment is attached to the wall of the pulmonary artery. The 

 inner border is free, and it presents at its centre a small nodule, the nodulus valvulae 

 semilunaris. On each side of the nodule there is a small, thin marginal segment, 

 of semilunar form, the lunule. Each segment of the valve is formed by a layer of 

 endocardium on its 'Ventricular surface, an endothelial layer of the inner coat of 

 the artery on its arterial surface, and an intermediate stratum of fibrous tissue. 

 Both the attached and the free margins of the cusps are strengthened by 

 fibrous bands, and strands of condensed fibrous tissue radiate from the outer 

 border of each cusp to the nodule, but they do not enter the lunulae. When the 

 valve closes the noduli are closely apposed, the lunulse of the adjacent segments 

 of the valve are pressed together, and both noduli and lunulae project vertically 

 upwards into the interior of the artery. 



The cavity of the right ventricle is lined with endocardium; the walls are 



smooth in the conus arteriosus, but are rendered rugose and sponge-like in the 



body by the inward projection of numerous muscular bundles, the trabeculse carneae. 



The fleshy trabeculse are of two kinds : the simpler are merely columns raised 



in relief on the wall of the ventricle ; the other class are rounded bundles, free 



in the middle, but attached at each end to the wall of the ventricle. One special 



bundle of the second group, called the moderator band, is attached by one extremity 



; to the septum, and by the other to 'the ster no-costal wall, at the base of the 



anterior papillary muscle ; it tends to prevent over-distension of the cavity. In 



addition to the trabeculae carneae conical muscular eminences, the musculi papillares, 



i project into the cavity of the ventricle. The bases of the papillary muscles are 



; continuous with the wall of the ventricle, and their apices 'terminate in numerous 



i chordae tendineae which are attached to the apices, the borders, and ventricular 



surfaces of the cusps of the tricuspid valve. 



The musculi papillares of the right ventricle are (1) a large anterior, muscle, 



from which the chordae pass to the anterior and inferior segments of the 



valve ; (2) a smaller and more irregular inferior muscle, sometimes represented 



by two or more segments, from which chordae pass to the inferior and medial 



i cusps; and (3) a group of muscular bundles, varying in size and number, 



which spring from the septum and are united by chordae to the anterior and 



i medial cusps. 



The walls of the right ventricle, the septal excepted, are much thinner than 

 those of the left, but the trabeculae carneae are coarser and less numerous in the 

 right than in the left ventricle. 



Ventriculus Sinister. The left ventricle is a conical chamber, and its cavity 

 is oval in transverse section. The base is directed upwards and posteriorly, and in 

 the greater part of its extent it is continuous with the corresponding atrium, 

 with which it communicates through the mitral orifice; but anteriorly and to % the 



