OCR Text |
Show 808 MR. J. B. PERRIN ON BALAENOPTERA ROSTRATA. [Dec. 6, the other, altogether precluded the possibility of such a membrane existing in these situations. The object of the laxity of the fibro-cartilage at the squamosal concavity was evidently to allow of the passage of a strong ligamentous band from the under surface of the cartilage to the depression in the squamosal, also to allow of the transmission of vessels. On the inner side the elastic fibro-cartilage projected beyond the squamosal, and was firmly attached to a strong zone or belt, of a more dense fibro-cartilaginous material, which surrounded the tympanic bone, affording to them au efficient retentive as well as protective nidus. On removing the articular mass, the tympanic zone could be well seen ; it was broader on the inner than on the outer side of the ossicle, and broader behind than in front. In the former situation it measured 2| inches from before backwards; in the latter 2 inches, whilst its width on the inner side was only one inch. In the centre of this nidus the oval under surface of the tympanic bone could be seen to tbe extent of two and a quarter inches, measured iu its long axis, and one inch and three-quarters measured transversely. The superficial or circumferential fibres of the articular cartilage at the maxillary end were prolonged forwards on to the circumference of the mandibular condyle, forming a capsular-like belt of attachment, thereby materially adding to the fixity of the cartilage. The fibro-elastic cartilage was still further protected by a dense ligamentous band which passed, from the squamosal to the maxillary condyle, along its under surface. Surrounding it was a dense mass of connective tissue, in which ramified an intricate plexiform network of vessels and nerves. Imbedded in this connective tissue, on the inner side, was a small wedge-shaped muscle, about 3| inches long, arising tendinous from the osseous ridge leading from the condyle to the inferior dental foramen. It was about half an inch wide, and terminated in a fascial expansion which was inserted into the fibro-cartilage upon its upper and inner side. The action of this muscle seems to have been to draw the articular mass forwards and inwards, thereby limiting the outward tendency it would otherwise have when the lower jaw was approximated to the upper, and also concentrating the cartilage in the axial line of the mandible during that movement. The cartilage weighed, when removed, 2\ lbs. Its circumference measured 17\ inches. Its maxillary extremity measured transversely 4 inches, and 2f inches from above downwards. Its squamosal end measured 5 inches laterally, and 4\ from above downwards. It presented the well-marked shape of the squamosal articular surface, namely concavo-convex, the concavity being situated antero-inter-nally, the convexity postero-externally. Its length was 4\ inches. This articulation is a true amphiarthrosis, exactly analogous to that between the vertebral segments of the human subject, minus the pulp. The muscles of mastication were moderately well developed, being short, thick, and strong, and so closely approximated to the articular |