552 PROF. E. R. LANKESTER ON THE HEART [June 20 
characteristically forms the right auriculo-ventricular valve in other 
Mammals, 1 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 satis- 
factory 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 incorresponding positions of the “standard” 
heart (that of Man) and of the heart of the Rabbit, which, curiously 
enough, differs more from that of Man 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 de- 
scribe 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.”” 
Tuer Rigut AURICULO-VENTRICULAR VALVE. 
A. Or Man.—From the tendinous margin of the right auriculo- 
ventricular orifice of the human heart depends into the ventricle a 
complete and continuous membranous collar (Pl. XX XVIII. figs. | 
& 2). Thisis 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 cusps’ (rac, Zac 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 (pc). 
The three cusps of the tricuspid valve of Man are attached by 
fine chords (the “‘ chordze tendinez ’’) to definite muscular lobes (the 
**musculi papillares *’) projecting from the ventricular wall, and also 
by some of the chorde directly to the ventricular wall. 
The two onterior 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. 1, @). The long 
? The “inferior” and “anterior” of human anatomists, In the present 
memoir the apex of the heart is regarded as inferior, the base as superior, the 
dorsal surface as posterior, the yentral surface as anterior. 
