556 PROF. E. R. LANKESTER ON THE HEART [JunC 20, 



papillary muscles, which should then be continued right through 

 the membranous collar of the valve to the auriculo-ventricular ring. 

 Further, we should have to suppose the suppression of the whole of 

 the septal division of the valve and its related chordae and muscles, 

 excepting a little piece in immediate relation with the right anterior 

 papillary muscle. 



The heart No. 2 (fig. 13) of Ornithorhynchus differs from No. 1 

 in the fact that there is not a single right anterior papillary muscle, 

 but five separate muscular slips representing it, of which three (fig. 13, 

 b, b, b) are attached to and traverse the membranous substance of 

 the valve, whilst two (m, n) pass over it and reach the auriculo-ven- 

 tricular ring. These latter are of great importance in the comparison 

 with the heart of Sauropsida, since they arise from the anterior 

 ventricular wall. The heart No. 2 is also remarkable, as already 

 mentioned, for the reduction of the septal portion of the membranous 

 valve to a vanishing quantity (fig. 1 &p c). 



In both hearts the three muscles of the valve (or two and the 

 subdivided third) become continuous with one another at their inser- 

 tion into the auriculo-ventricular ring. The large papillary muscle 

 in the middle spreads out on either side, and on the left completes the 

 arch formed by the muscular lobe (e) rising from the side of the pul- 

 monary cone; on the right it similarly completes an arch, of which 

 the right side is formed by the spreading insertion of the right papil- 

 lary muscle or its representative shps (figs. 12, 13). 



It is thus quite clear that the description of the right auriculo- 

 ventricular valve of Ornithorhynchus as " fleshy " is quite correct ; 

 membrane, though present, plays a subordinate part as compared 

 with what is seen in other Mammalia. 



Not only this, but it seems probable, from the very imperfect 

 development of a septal or posterior flap to the valve, that the action 

 of the valve must differ importantly from that of the valve of other 

 mammals, and resemble that of the fleshy valve of birds and reptiles. 



The reflux of blood into the auricle on contraction of the ventricle 

 is not prevented in Ornithorhynchus by a passive floating-out of 

 membranous cusps, but, as in birds and reptiles, the muscular arches 

 of the valve are by their active contraction pressed against the septal 

 portion of the auriculo-ventricular orifice, which is for two thirds (or 

 sometimes more) of its extent devoid of any depending cusp or mem- 

 branous flap. 



How far there is a real agreement, due to common inheritance 

 from a common ancestor, between the muscular structures of the 

 right cardiac valve of Ornithorhynchus and those of the similarly 

 placed valve in birds and certain reptiles, is a distinct and very 

 important question, into the discussion of which I do not propose 

 to enter on the present occasion. 



The Left Auriculo-ventricular Valve. 



The valve which is called " mitral " in Man's heart differs in struc- 

 ture in Ornithorhynchus from the human standard, although much 

 less so than does the right auriculo-ventricular valve. 



