OCR Text |
Show 1882.] OF ORNITHORHYNCHUS PARADOXUS. 551 apex directed backwards) in a nearly horizontal direction in the valve. They occupy, however, scarcely the third part of the entire length of this division of the valve. At the broadest part of this same division a muscular band passes from the ventricular septum, and is inserted into the former, and spreads its fibres in a fan-like expanse in the valve nearly up to the origin of the valve from the margin of the ostium. A second smaller bundle lies behind this, also arising from the septum. If we spread out the valve, and compare the purely membranous surface with that provided with muscular tissue, the former is found to be larger than the latter. The second portion of the valve arises from the part of the ostium belonging to the septum. At its narrower part it is in continuity with the other division of the valve, broadens out from behind forwards, and is fastened to the septum along a perpendicular line stretching from the ostium into the ventricle. It is therefore not only fastened to the circumference of the ostium, but, starting from there, also to the septum. Since the latter line of fixation is perpendicular to the line of origin along the ostium, this portion of the valve forms a ' pocket-valve '-the more so since no trabeculae pass to its free margin, and moreover no muscular fibres can be detected in its substance. " When a comparison of this arrangement is made with that of Birds, the difficulty is at once obvious that in Ornithorhynchus the septal portion of the ostium has a valve, whilst such is wanting in Birds. The whole apparatus cannot, therefore, be compared with that of Birds, but only the portion of the valve which arises from the outer half of the circumference of the ostium." In the absence of figures it is not possible fully to comprehend Professor Gegenbaur's description ; but it seems to me probable that the heart examined by him differed individually from those studied by Meckel and Cuvier, and from the two examined by me. In these two, as will be seen below, considerable differences were observed on comparison one with another. The main point on which Gegenbaur insists, is the existence of a septal portion to the valve ; it is on this account that he objects to a comparison with the Bird's valve. But this septal portion seems to have been exceptionally large in the heart studied by him. In both my specimens it was small, and left the larger part of the septal margin of the ostium unprovided with any valvular fold. At the same time it was larger in one specimen than in the other. The existence of a greater or less portion of the valve along the septal side of the ostium does not appear to invalidate the comparison of the main bulk of the valvular structure with that of the Bird's heart, though the closeness of the agreement is diminished by the fact insisted on by Gegenbaur, viz. that the muscular bands of the valve arise in Ornithorhynchus, as in the Crocodile, from the septal wall of the ventricle, and not from the free outer wall as in the Birds. Recognizing, as all anatomists must do, the great interest attaching to the observation that in Ornithorhynchus muscular tissue to a large extent invades and replaces the membranous structure which P R O C ZOOL. Soc-1882, No. XXXVII. 37 |