1882.] OF ORNITHORHYNCHUS PARADOXUS. 553 



axis of this lobe would, if produced, pass between the two anterior 

 cusps in the notch which separates them. Right and left two groups, 

 each of four or more chordae tendinese, pass from the free end of this 

 papillary muscle, and, spreading out fan-wise, join respectively the 

 right and the left cusps of the membranous valve ; so that the adjacent 

 margins of the two cusps are connected by groups of chordae to the 

 papillary muscle ; and were the groups of chordae to be enlarged 

 and fuse with one another, we should have in place of a right and 

 a left anterior cusp one large anterior cusp connected by the most 

 prominent portion of its border to the papillary muscle. This hypo- 

 thetical condition is realized in Ornithorhynchus. 



The large papillary muscle thus related in Man to the right and 

 left anterior cusps of the tricuspid valve, is the " great" or " anterior " 

 papillary muscle. 



The right border or curvature of the right anterior cusp is con- 

 nected by chordae tendinese to a small papillary muscle, which may 

 be called the '* right papillary muscle" (figs. 1 & 2, b). 



The left border or curvature of the left anterior cusp is connected 

 by chordae directly to the wall of the ventricle, to the right of that 

 region which is known as the pulmonary cone (figs. 1 & 2,e). 



The septal cusp or segment of the tricuspid valve of Man may be 

 regarded as really the posterior equivalent of both anterior cusps, 

 not divided or produced into two' pieces. It is connected to the 

 septal wall of the ventricle directly by chordae tendineae, and also to 

 two small papillary muscles which project from that wall (fig. 2, c, d), 

 and may be known as the posterior or septal papillary muscles 

 (greater and less). These are smaller than the right papillary 

 muscle ; and all the other papillary muscles are much smaller than 

 the "great" or "anterior" papillary muscle. 



B. Of the Rabbit. — Whilst a large number of mammals possess 

 a tricuspid valve very closely similar in its arrangements to that of 

 Man (see PI. XLI. figs. 18-22), in the Rabbit (PI. XXXVIII. 

 figs. 3, 4) a considerable divergence from the human standard occurs. 



The valve of the right side of the Rabbit's heart cannot be de- 

 scribed as " tricuspid " in anj"^ sense. It is a continuous membranous 

 collar connected by numerous chordae tendineae to two rows of musculi 

 papillares. The valvular collar is elliptical in form ; and it may be 

 divided into an anterior and a posterior (or septal) portion corre- 

 sponding to the two long sides of the ellipse. 



The two rows of musculi papillares are arranged along the septal 

 portion of the ventricular wall as a superior and an inferior series. 

 The superior series are connected by chordae tendineae with the 

 posterior or septal half of the valve ; the inferior series are connected 

 by chordae with the anterior half of the valve. 



There are seven musculi papillares in the inferior series belonging 

 to the anterior part of the valve '. Those of the superior series, belong- 



^ Since the above was written I have found that great variations occur in the 

 points described in indi\idunl Eabbits. Often the musculi papillares are not 

 more than three in number, as in the figure of a Rat's ventricle (Plate XXXIX. 

 fig. 11). 



37* 



