OCR Text |
Show 1899.] OSTEOLOGY OF THE TUBINARES. 395 trigeminal foramen lies above the internal and auditory meatus, betAveen the prootic and alisphenoid. The opisthotic, seen from without, is largely cartilaginous. Ossification has, however, begun in the shape of a crescentic nodule of bone lying immediately beloAv the fenestra ovalis and above the vagus foramen. Inside it serves to divide the exoccipital and prootic. Ossification has begun from two centres-caudad behind the vagus foramen, laterad of the extreme upper end of the exoccipital and at the base of the epiotic ; and anteriorly, as a small nodule in the region corresponding with a line continuing the exoccipital suture outwards to the postero-ventral border of the prootic. Both ossifications have fused with the prootic, but remain separate from the exoccipital. The basisphenoid is not visible from without, being concealed by the basitemporal plate. Inside it is bounded, caudad by the basioccipital, laterad by a tract of cartilage Avhich divides it from the alisphenoid, and in the middle line, in front, by the cartilaginous presphenoid. It forms, with the basioccipital, the anterior region of the metencephalic fossa. The pituitary fossa is lodged in its anterior border, and is yet only a shallow depression with an aperture in its posterior border for the internal carotid canal. A mass of diplce divides the basisphenoid from the parasphenoid below. The alisphenoid is more or less quadrate in form-viewed from without. Its outer border is convex and received into the concave border of the squamosal (Pl. XXIII. fig. 1). It is bounded above by the orbital plate of the frontal, below by the lateral wing of the parasphenoid (p. 384) and the trigeminal foramen. The orbitosphenoid is represented only by a sheet of membrane. The presphenoid is still cartilaginous and continuous in front with the mesethmoid. The mesethmoid is a vertical liuguiform plate resting upon the distal end of the parasphenoid below and underlying the nasals above. Its posterior border is semicircular, its anterior slopes obliquely backwards and terminates at the free end of the nasal process of the premaxilla. The olfactory cavity, seen from the inside, after the removal of the mesethmoid, contains a large posterior and an elongated ventral accessory turbinal. The quadrate does not differ materially from that of the adult. The otic and squamosal processes are somewhat less distinctly marked. The columella has well-marked extra- and infrastapedial rays • the suprastapedium is very short. The articulare can still just be distinguished as a separate element (fig. 1, p. 392). The Membrane-bones. The parietal is trapezoid. Its outer, anterior, and mesial |