OCR Text |
Show 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 tendineae, 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 hypothetical condition is realized in Ornithorhynchus. The large papillary muscle thus related in M a n 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 connected by chordae tendineae 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 M a n 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. O F T H E RABBIT.-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 described as " tricuspid " in any 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 corresponding 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- 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 ventricle (Plate X X X I X. fig" U)- 37* |