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Show 558 PROF. E. R. LANKESTER ON THE HEART [June 20, two auricles. This, however, proved to be au erroneous anticipation. The extension of this angle of the right auricle, und the constriction into the form of an oval ring of the communication between it and the main cavity of the auricle, are sufficiently remarkable. The site of the interauricular communication is not marked in the adult Ornithorhynchus by a fossa. EXPLANATION OF THE PLATES. PLATE XXXVIII. Fig. 1. M A N . View of the tricuspid valve as seen on removing the anterior wall of the right ventricle. a, great anterior papillary muscle ; b, right or lesser anterior papillary muscle ; c, greater septal or posterior papillary muscle; a, lesser septal or posterior papillary muscle; e, point of attachment of left anterior chorda; tendinea;, not developed as a papillary muscle; r a c, right anterior cusp of the membrane of the valve ; lac, left anterior cusp of the membrane of the valve. 2. M A N . The same view, but the great papillary muscle is now cut through, and the right and left anterior cusps of the valve are reflected so as to expose the auriculo-ventricular orifice and the septal cusp. N.B. A dark background is introduced behind the chorda; tendineae of the reflected cusps, by inadvertence of the lithographer. Letters as in fig. 1, excepting a', apex of the great papillary muscle, cut away from a. p c, posterior or septal cusp of the membrane of the valve. 3. R A B B I T (Lepus cuniculus). View of the " tricuspid " valve similar to that given in fig. 1, showing the seven anterior papillary muscles, of which a, a, a represent the great papillary muscle of Man, whilst b, b, b, b represent the right or lesser anterior papillary muscle of Man, e corresponding with e in the human heart. P. A, pulmonary artery. 4. The same heart with the anterior papillary muscles cut through and the valve reflected, exposing the attachments c, d of the posterior or septal flaps, a', a', a', apices of the larger anterior papillary muscles cut away from their bases and reflected ; b', b', b', apices of three of the smaller (right) anterior papillary muscles similarly cut away. Other letters as in fig. 3. 5 & 6. Diagrams of views of the basal aspect of the heart of Ornithorhynchus (fig. 5) and the Rabbit (fig. 6), intended to show the relative proportions and form of the right and left auricles, and especially the position of the csecal appendix (Ces) of the right auricle of Ornithorhynchus. Letters as in fig. 7. PLATE XXXLX. Fig. 7. OUNITIIORIIYNCIITJS. Anterior (or, more correctly, ventral) aspect of the heart. 7 a. Lateral view of same heart, right side. 7b. Posterior (or, more correctly dorsal) aspect of the same heart. 7 c Lateral view of same heart; left side. Letters in figs. 5 to 8:-E. V. C. S, right vena cava superior; It. A, right auricle; E. V, right Ventricle; L. V. C. S, left vena cava superior; L. A, left auricle; L. V, left ventricle; P. A, pulmonary artery ; P. V, pulmonary vein; Ao, aorta; V. C. I, vena cava inferior. 8. Dorsal aspect of the same heart, the right auricle being opened and the cut walls reflected, in order to show the inner surface. Ca, oval orifice of the csecal appendix, mistaken hitherto for a fossa ovalis; F. 0, position of fossa ovalis in hearts of Placentalia; V. C. I, orifice of the vena cava inferior; L. V. C. S, orifice of the left vena cava superior; E. auriculo-ventr Vic.u Cla.r aSp,e rotruirfei.ce of the right vena cava superior; A. V. a, |
