1882. ] OF ORNITHORHYNCHUS PARADOXUS. 553 
axis of this lobe would, if produced, pass between the two anterior 
cusps in the notch which separatesthem. Right and left two groups, 
each of four or more chorde tendinez, 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 chord to the 
papillary muscle; and were the groups of chord 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 chord tendinese to a small papillary muscle, which may 
be called the “ right papillary muscle”’ (figs. 1 & 2, 6). 
The left border or curvature of the left anterior cusp is connected 
by chordee 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 chord tendinez, and also to 
two small papillary muscles which project from that wall (fig. 2,¢, 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. Or roe Rassir.—Whilst a large number of mammals possess 
a tricuspid valve very closely similar in its arrangements to that of 
Man (see Pl. XLI. figs. 18-22), in the Rabbit (Pl. 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 any sense. Itis a continuous membranous 
collar connected by numerous chord tendineze 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 chorde tendinee with the 
posterior or septal half of the valve ; the inferior series are connected 
by chordze 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- 
1 Since the above was written I have found that great variations occur in the 
points described in individual Rabbits. Often the musculi papillares are not 
more than three in number, as in the figure of a Rat’s yentricle (Plate XXXIX. 
fig. 11). 
37* 
