652 PROF. E. R. LANKESTER ON THE HEART [JunC 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 incorrespondingpositionsof 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 Ornithorhi/nchus ; 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. Of 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 (PI. XXXVIII. figs. 1 

 & 2). This is tbe 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* {r a c, lac 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 " chordse tendineas ") to definite muscular lobes (the 

 "musculi papillares ") projecting from the ventricular wall, and also 

 by some of the chordse 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 



^ 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 ventral surface as anterior. 



