OCR Text |
Show 1891.] ANATOMY OF ORNITHORHYNCHUS. 581 by Turner (4), that in this animal the premaxillary bone consists of two entirely distinct and separate portions, an outer and an inner. Of these two the inner unites with its fellow of the opposite side to form a small dumb-bell-shaped bone, I'os paradoxe of Albrecht. As the Ornithorhynchus is the only mammal in which this arrangement of the premaxillary bone exists, it appeared to me that its position and relations were worthy of a more detailed examination than they have yet received. J. F. Meckel and Owen have described its general form when looked at from below, while Albrecht and Turner have shown its relations to the naso-palatine foramen. I propose now to demonstrate from my serial sections its relations to the cartilages of the nose and to Jacobson's organ. When looked at from below, this bone is seen to consist of two rounded nodules placed one in front of the other and united by a narrow portion. Of the two nodules, the anterior is the smaller. The naso-palatine foramen lies just external to the constricted portion. Its total length in the adult is about 7 mm., and the breadth of the posterior swelling 5 m m. The anterior nodule does not reach quite so far forward as the nostrils, but extends a little beyond the anterior end of Jacobson's organ. On transverse section (see fig. 1, PI. XLIIL) it is seen to form a thin layer of bone, flattened from above downwards. It is separated from the cavity of the mouth by the mucous membrane of the hard palate and some loose connective tissue, while its upper surface lies in close relation to the cartilages of the nose which contain Jacobson's organ. As it passes backwards it gets thicker and develops a median ridge on its upper surface, which passes upwards between the two plates of cartilage containing Jacobson's organ. Opposite the naso-palatine foramen its vertical extent is about twice that of its transverse, and its external surfaces are concave and lodge the inner part of the cartilage of Jacobson's organ. A section through about the middle of the posterior nodule of the bone shows that it possesses the same general form as at the naso-palatine foramen. It appears here as composed of two crescents with their convexities directed inwards, and the greater breadth of the bone is due to the elongation of the horns of the crescents, which reach about halfway round the organ of Jacobson and are in close contact with its cartilage. In the greater part of its extent the bone is covered on its oral aspect by mucous membrane and a thin layer of submucous tissue, but near its posterior extremity also by a layer of cartilage. This is effected by an extension inwards to the middle line of the plates of cartilage which at the naso-palatine foramina lie external to these openings. The dumb-bell-shaped bone from its position in relation to the cartilages of the nose is evidently ossified in the membrane investing them. In some places the ossific process has involved the whole of the tissue up to the cartilage, so that the bone and cartilage are in direct contact; as a rule, however, there is a thin layer of connective tissue between the bone and cartilage. |