558 PROF. E. R. LANKESTER ON THE HEART [Juue 20, 



two auricles. This, however, proved to be an erroneous anticipation. 

 The extension of this angle of the right auricle, and 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 OP THE PLATES. 



Plate XXXVIII. 



Fig. 1. Man. View of the tricuspid valve as seen on removing the anterior 

 wall of the right venti-icle. 



a, great, anterior papillary muscle ; b, right or lesser anterior 

 papillary muscle ; c, greater septal or posterior papillary muscle ; 

 d, lesser septal or posterior papillary muscle ; e, point of attachment 

 of left anterior chord* teudineje, not developed as a papillary muscle ; 

 r a c, right anterior cusp of the membrane of the valve ; lac, left an- 

 terior cusp of the membrane of the valve. 



2. Man. 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-veutricnlar orifice and the septal cusp. 



N.B. A dark background is introduced behind the chordio teudinese 

 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. Rabbit {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', 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 Ornitho- 

 rhynchus (fig. 5) and the Eabbit (fig. 6), intended to show the 

 relative proportions and form of the right and left auricles, and 

 especially the position of the csecal appendix ( Ccb) of the right auricle 

 of Ornithorhynchus. Letters as in fig. 7. 



Plate XXXLX. 

 Fig. 7. ORNiTHORHYNCTtrs. Antei'ior (or, more correctly, ventral) aspect of 



the heart. 

 7 a. Lateral view of same heart, right side. 



76. 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 : — B. V. C. S, right vena cava superior ; E. A, 



right auricle ; S. V, right ventricle ; L. V. C. 8, left vena cava superior ; 



L. A, left auricle ; L. V, left ventricle ; P. A, pulmonary artery ; 



P. V, pulmonary vein ; Ac, 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. Cte, oval 



orifice of the ceecal appendix, mistaken hitherto for a fossa ovalis ; 



F. 0, position of fossa ovalis in hearts of Placentalia ; F. C. I, orifice 



of the vena cava inferior ; L. V. C. 8, orifice of the left vena cava 



superior ; E. V. C. 8, orifice of the right vena cava superior ; A. V. a, 



auriculo-ventricular aperture. 



