558 PROF. E. R. LANKESTER ON THE HEART [June 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 OF THE PLATES. 
Prats XXXVIII. 
Fig. 1. Man. View of the tricuspid valve as seen on removing the anterior 
wall of the right ventricle. 
a, great anterior papillary muscle; 6, right or lesser anterior 
papillary muscle; ¢, greater septal or posterior papillary muscle ; 
d, lesser septal or posterior papillary muscle; e, point of attachment 
of left anterior chords tendinex, voz developed as a papillary muscle ; 
rae, right anterior cusp of the membrane of the valve ; Jae, 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-ventricular orifice and the septal cusp. 
N.B. A dark background is introduced behind the chord tendinese 
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. pc, posterior or septal cusp of the membrane of 
the valve. 
. Rassrr (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 
2, b, b, b represent the right or lesser anterior papillary muscle of Man, 
e corresponding with ¢ in the human heart. P. 4, 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. 5. 
5 & 6. Diagrams of views of the basal aspect of the heart of Ornitho- 
rhynchus (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 cxcal appendix (Ce) of the right auricle 
of Ornithorhynchus. Letters as in fig. 7. 
Puiate XXXIX. 
Fig.7. Orniruoruyncnvs. Anterior (or, more correctly, ventral) aspect of 
the heart. 
7a. Lateral view of same heart, right side. 
7b. Posterior (or, more correctly dorsal) aspect of the same heart. 
Tc. Lateral view of same heart ; left side. 
Letters in figs. 5 to 8:—R. V. C. S, right vena cava superior; FR. A, 
right auricle; 2. V, right ventricle; L. V. CS, left vena cava superior ; 
L. A, left auricle; L. V, left ventricle; P.A, pulmonary artery ; 
P. V, pulmonary vein; 4o, aorta; V. C.J, 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. Ce, oval 
orifice of the cecal appendix, mistaken hitherto for a fossa ovalis ; 
F. O, position of fossa ovalis in hearts of Placentalia; V. C. J, orifice 
of the vena cava inferior; L. V. C. 8, orifice of the left vena cava 
superior; R. V. C. S, orifice of the right vena cava superior; 4.V. a, 
auriculo-ventricular aperture. 
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