OCR Text |
Show 260 ON THE STUUOTUBE OF THE SKUJ_,L, the skull that the lambdoidal suture is not very distant frmn its summit. The plane of the roof of the sku~l sl~pes upwards and forwards, fron1 the occipital foran1en to .tlus po1nt. Fig. 101.-The skull of a young Echidna viewed from without, and in lrJngitudinal ancl yertical section. rrhe large parietals, anchy losed together in the middle line, form all but a very small portion of the rest of the roof of the skull, and are succeeded by the small frontals. These are met in the middle line, inferiorly, by the lamina perpend,icularis of the ethmoid, which separates one olfactory chan1ber from the other, and are united by sutures, anteriorly, with the long nasals. These stop short of the anterior nasal aperture, being excluded therefrom by the premaxillaries. In the base of the skull the basi-sphenoid, presphenoid, and ethmoid are anchylosed together. The basi-sphenoid is a wide, flattened bone, somewhat deflexed at the sides. Its long, thin, postero-lateral margins articulate externally with the broad, flat bones ( Pt) which contribute above to form the floor of the cranial cavity by filling up a vacuity which would otherwise exist between the basi-sphenoid, periotic, and alisphenoid. The thick posterior and external edges of these bones are exeavated by a deep groove, which forms the front wall of the tympanum and of the Eustachian tubes. rrhe palatine bones are completely anchylosed with the sphenoid, and pass abruptly inward:s fro1n the outer edges of that bone (Fig.102). The anterior n.nd internal edges of THE SKULLS OF MAMMALIA. 261 the bones ( Pt), which obviously represent the pterygoids, articulate with them as well as with the basi-sphenoid. The anterior and external edges of the pterygoids are united with an anterolateral prolongation of the pro-otic part of the periotic; and, rather above the cleft between the latter and the pterygoid, is fixed the large process of the malleus (m, Fig. 102), to which the tympanic ring closely adheres. The periotic bone is remarkable for the la1nellar prolongations which it sends forwards from its pro-otic, epiotic, and opi:sthotic regions, beyond the space required for the auditory organ, and which enter more largely into the side walls of the skull than any of its ordinary constituents. The periotic contributes towards the floor of the skull by a triangular process, which it sends in between the basi-occipital and the basi-sphenoid. Posteriorly, it articulates largely with the ex-occipital, the foramen Fig. 102. for the eighth pair being situated between it and the latter. By its wide superior prolongation it unites behind with the ex -occipital, posteriorly and superiorly with the supra-occipital; anteriorly and superiorly, first with the parietal, and then with a large bone ( 08, Fig. 101) which stretches outwards, upwards, and backwards from the presphenoid and ethn1oid, articulating partly with the frontal, and more extensively with the parietal. Except in its unusual articulation with tpe periotic, this bone corresponds with the orbito-sphenoid. Between the superior prolongation of the periotic, and its thin and imperfectly-ossified anterior and inferior prolongation, there is an interspace filled up by the squamosaL · The lower edge of this prolongation articulates with the pterygoid, and, in front of this, fonns the upper boundary of the foramen for the third division of the fifth Fig. 102. - Under view of the left nerve. Between its front edge and a small half of the skull of process, sent up by the palatine towards the Echidna. orbito-sphenoid, is a small plate of bone, which alone seems to represent the alisphenoid. |