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Show 552 PROF. E. R. LANKESTER ON THE HEART [June 20 characteristically forms the right auriculo-ventricular valve in other Mammals, I have taken an opportunity of carefully examining and drawing the valves and some other structural features seen in the hearts of two specimens of Ornithorhynchus presented to me by my friend Professor Liversidge, of Sydney, Australia. The main object of the present communication is to publish satisfactory illustrations, with explanatory description, of the appearances presented by these two hearts. I cannot doubt that both anthro-potomists and zoologists will be glad to possess something like a sufficient record of the very important facts observable in the heart of Ornithorhynchus; and I have added for comparison drawings of identical dissections viewed in corresponding positions of the " standard" heart (that of Man) and of the heart of the Rabbit, which, curiously enough, differs more from that of M a n in respect of the structure of its right auriculo-venticular valve than does that of the sheep, the ox, the dog, the hedgehog, the great ant-eater, the wombat, and the koala, which I have examined with especial reference to this point, and some of which are illustrated in the plates. For several interesting hearts I am indebted to the kindness of Mr. W . A. Forbes, Prosector to the Society. I shall first describe the figures accompanying this paper which illustrate the comparative structure of the right auriculo-ventriculai valve of Man, the Rabbit, and Ornithorhynchus; I shall then describe the left auriculo-ventricular valve of Ornithorhynchus ; and finally point out some peculiarities in the structure of the auricles of the heart of that animal, which have led to the erroneous statement that it possesses a deeply marked " fossa ovalis." THE RIGHT AURICULO-VENTRICULAR VALVE. A. O F MAN.-From the tendinous margin of the right auriculo- • o o ventricular orifice of the human heart depends into the ventricle a complete and continuous membranous collar (Pl. XXXVIII. figs. 1 & 2). This is the so-called tricuspid valve. It derives its name from the fact that, although forming one continuous collar-like ring, the membrane is produced at three points, forming three cusps or flaps. Two of these cusps are anterior in position, and may be called right and left anterior cusps1 (r a c, I a c in the figures). The third is wider than the two anterior, and rests against the septum or wall separating right from left ventricle; it may be called the posterior or septal cusp (p c). The three cusps of the tricuspid valve of Man are attached by fine chords (the " chordae tendineae ") to definite muscular lobes (the "musculi papillares ") projecting from the ventricular wall, and also by some of the chordae directly to the ventricular wall. The two anterior cusps of the valve are in relation with the largest muscular lobe or musculus papillaris, which springs from the septal surface near the apex of the ventricular chamber (fig. I, a). The long 1 The "inferior" and "anterior" of human anatomists. In the present memoir the apex of the heart is regarded as inferior, the base as superior, tbe dorsal surface as posterior, the ventral surface as anterior. |