2 
CHAMBERS OF THE HEART. 745 
consists of a double fold of endocardium, strengthened by a little intermediate 
fibrous tissue, and the bases of the cusps are generally continuous with each other 
at the auriculo-ventricular orifice, where they are attached to a fibrous ring, 
but they may be separated by small intermediate lobes which fill the angles 
between the main segments. The apices of the cusps hang down into the 
ventricle. The margins, which are thinner than the central portions, are notched 
and irregular. The auricular surfaces are smooth. The ventricular surfaces are 
roughened, and, like the margins and apices, they give attachment to fine tendin- 
ous cords, the chorde tendinex, the opposite extremities of which are attached to 
muscular bundles, the musculi papillares, which project from the wall into the 
cavity of the ventricle. 
The pulmonary orifice, which lies in front and to the left of the tricuspid orifice, 
is guarded by a pulmonary valve composed of three semilunar segments (valvulee 
semilunares a. pulmonalis), 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 corpus Arantii (nodulus valvule semilunaris), and on each side of 
this body a small thin marginal segment of semilunar form, the lunule (lunula 
valvulze semilunaris). Each segment of the valve is formed by a layer of 
endocardium on its ventricular surface, a 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 tendinous bands, and strands 
of condensed fibrous tissue radiate from the outer borders to the corpora Arantu, 
but they do not enter the lunule. When the valve closes the corpora Arantil 
are closely apposed, the lunule of the adjacent segments of the valve are pressed 
together, and they project vertically upwards into the interior of the artery. 
The cavity of the right ventricle is lined by 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 columne 
carnee (trabecule carne). These bundles are of three kinds; the simplest are 
merely columns raised in relief on the wall of the ventricle; those of the second 
class are rounded bundles, free in the middle, but attached at each end to the wall 
of the ventricle. One special bundle of this group, called the moderator band, 1s 
attached by one extremity to the septum, and by the other to the antero-superior 
wall, at the base of the anterior papillary muscle ; it tends to prevent over-distension 
of the cavity. The third group of columne carnee are the musculi papillares, 
conical bundles continuous at their bases with the muscular wall of the ventricle, 
and terminating at their apices in numerous chord tendinew 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 chord pass to the infundibular and marginal segments of the 
valve; (2) a smaller and more irregular posterior muscle, sometimes represented by 
two or more segments, from which chord pass to the marginal and septal cusps ; 
and (3) a group of muscular bundles, varying in size and number, which spring 
from the septum and are united by chordee to the infundibular and septal cusps. 
The walls of the right ventricle, the septal excepted, are much thinner than 
those of the left, but the column carne of the first and second classes are coarser 
and less numerous in the right than in the left ventricle. 
The left ventricle (ventriculus sinister) is a conical chamber, and its cavity 1s 
oval in transverse section. The base is directed upwards and backwards, and in the 
greater part of its extent it is continuous with the corresponding auricle with 
which it communicates through the mitral orifice, but in front and to the right of 
its communication with the auricle it is continued into the ascending aorta. 
The mitral orifice is oval; its long axis runs obliquely from above and to the 
left downwards and to the right, and it is guarded by a valve consisting of 
two cusps, which is known as the mitral valve (valvula bicuspidalis). The two 
cusps of the valve are triangular and of unequal size. The smaller of the two, 
placed to the left and behind, is named the marginal, and the larger, placed to the 
right and in front, between the mitral and aortic orifices, 1s known as the aortic 
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