OCR Text |
Show 556 PROF. E. R. LANKESTER ON THE HEART [June 20, papillary muscles, which should then be continued right through the membranous collar of the valve to the auriculo-ventricular ring. Further, we should have to suppose the suppression of the whole of the septal division of the valve and its related chordae and muscles, excepting a little piece in immediate relation with the right anterior papillary muscle. The heart No. 2 (fig. 13) of Ornithorhynchus differs from No. 1 in the fact that there is not a single right anterior papillary muscle, but five separate muscular slips representing it, of which three (fig. 13, b, b, b) are attached to and traverse the membranous substance of the valve, whilst two (m, n) pass over it and reach the auriculo-ventricular ring. These latter are of great importance in the comparison with the heart of Sauropsida, since they arise from the anterior ventricular wall. The heart No. 2 is also remarkable, as already mentioned, for the reduction of the septal portion of the membranous valve to a vanishing quantity (fig. 16 pc). In both hearts the three muscles of the valve (or two and the subdivided third) become continuous with one another at their insertion into the auriculo-ventricular ring. The large papillary muscle in the middle spreads out on either side, and on the left completes the arch formed by the muscular lobe (e) rising from the side of the pulmonary cone; on the right it similarly completes an arch, of which the right side is formed by the spreading insertion of the right papillary muscle or its representative slips (figs. 12, 13). It is thus quite clear that the description of the right auriculo-ventricular valve of Ornithorhynchus as " fleshy " is quite correct; membrane, though present, plays a subordinate part as compared with what is seen in other Mammalia. Not only this, but it seems probable, from the very imperfect development of a septal or posterior flap to the valve, that the action of the valve must differ importantly from that of the valve of other mammals, and resemble that of the fleshy valve of birds and reptiles. The reflux of blood into the auricle on contraction of the ventricle is not prevented in Ornithorhynchus by a passive floating-out of membranous cusps, but, as in birds and reptiles, the muscular arches of the valve are by their active contraction pressed against the septal portion of the auriculo-ventricular orifice, which is for two thirds (or sometimes more) of its extent devoid of any depending cusp or membranous flap. H o w far there is a real agreement, due to common inheritance from a common ancestor, between the muscular structures of the right cardiac valve of Ornithorhynchus and those of the similarly placed valve in birds and certain reptiles, is a distinct and very important question, into the discussion of which I do not propose to enter on the present occasion. THE LEFT AURICULO-VENTRICULAR VALVE. The valve which is called " mitral " in Man's heart differs in structure in Ornithorhynchus from the human standard, although much less so than does the right auriculo-ventricular valve. |