OCR Text |
Show 1882.] OF ORNITHORHYNCHUS PARADOXUS. 555 papillaris, although not "papillary" (figs. 12 & 13, a). It obviously corresponds to the great or anterior papillary muscle of the human heart. The vertex of the triangle formed by the membranous flap of the valve is bisected by this great papillary muscle, which widens out as it passes upwards, and is inserted into the anterior border of the auriculo-ventricular ring, thus dividing the membranous triangular flap into two distinct pieces, a right and a left. It is therefore perhaps not quite correct to speak of these two segments united by the muscular band as one anterior flap : the flap might perhaps be regarded as composed of two membranous segments united by a median muscular band which is the prolonged anterior papillary muscle. The membranous substance of the valve, however, is distinctly continued beneath or on the lumen side of the muscular band, thus uniting the two halves, which at first sight seem to be separated by the muscular substance. Obviously the right segment corresponds to the right anterior cusp, and the left segment to the left anterior cusp, of the human tricuspid valve. The peculiarity of this region in Ornithorhynchus consists in the fact that there are no chordae tendineae connecting the membranous segments to the great papillary muscle, while the muscle itself is attached directly to the membranous flap, and is continued through it up to the auriculo-ventricular ring, so as to invade (over a broad band-like area) the membrane of the valve by muscular tissue. There is also a complete absence of chordae tendineae from the left border of the left anterior flap, which is, in their absence, directly attached to a fleshy arch which extends from the side of the pulmonary cone up to the auriculo-ventricular ring, where it is confluent with the expanded insertion of the great papillary muscle (figs. 12, 13,14,15, e). Consequently the left membranous segment of the valve is triangular in shape, the free border forming the base of an isosceles triangle. Both hearts examined agreed in the features so far described. In regard to the connexions of the right membranous segment of the anterior portion of the valve, they differ a little from one another. In heart No. 1 (fig. 12) there is a well marked right papillary muscle (b) corresponding to the similarly placed muscle in Man, but differing from that of Man, just as does the great papillary muscle, in the fact that it is devoid of any chordae tendineae and is continued from the ventricular wall up to the auriculo-ventricular ring, having the right border of the right membranous cusp or segment attached directly to its left border in the upper part of its course. Septal flap.-To the right or posterior border of this smaller muscular band in heart No. 1, one lateral border of the very small septal membranous flap is attached. This rudimentary posterior or septal flap arises along the posterior or septal margin of the auriculo-ventricular orifice for not more than one third of the extent of that margin, the rest being free from any valvular collar or fringe (figs. 14 & 15, p c). The condition seen in heart No. 1 may be understood by supposing, in a valve arranged as in Man, the chordae tendineae to become muscular and compacted together and so to form parts of the |