OCR Text |
Show 170 ON '!'HE S'l'HUCTURE OF 'l'HE SKULL. Furthermore, in the footal human skull, the basi-sphenoid contributes nothing towards the posterior boundary of tho pituitary fossa, which is formed by the long cartilaginous synchondrosis which connects the rudimentary basi-sphenoid with the basi-. occipital. I identify the lamella of cartilage which I have described in the Pike with this synchondrosal portion of the footal human basis cranii. But the basi-occipital, basi-sphenoid, presr henoid, etlnnoid, and vomer of 1\fan being now accounted for in the Pike's craniofacial axis, what, it may be said, is tho nature of the bone x to which the tenn "basi-sphenoid " is co1nn1only applied? It differs from any of the os ifications of the basi-sphenoidal cartilage in Man, not only by extending ba kwards beneath the basi-occipital, but by stretching forvvard .. , beneath the pre~ sphenoidal and ethmoidal cartilag s, to within a short distance of the anterior extremity of the cranium; and in the still more important circumstance that it is an o ification within the perichondrium, which can be stripped off, in skulls which have been Inacerated, or steeped for a hort time in boiling water, without injury to the cartilage upon whi h it is d voloped. 1vlr. Parker has shown, in hi valuable paper on Balmniceps,* that the so-called basi- phonoid f birds is developed from three ossifications, a central one, the true ba i-sphenoid, and two lateral and in£ rior centres-the "basi-tmnpo1·als" (Parker), which appear to correspond with the lingulm of 1\ian. The thought readily ari. es that the single bone x may correspond with these two ba i-t mporal o" ifications. The latter, however, appear to be cartilage o ifications like the lingulte themselves: and, upon the \ hol , I think it will be safer, at any rate for the present, to regard the bone x a peculiar to the branchiate Verteb1·ata, and to confer upon it the special name of "parasphenoid." Connected with the bone of the basi-cranial axis are upper arche , and, as in 1\fan, th hindermo t of these arches consistR of three elements, two of which arc lat rnl and one superior. Each lateral bone articulat s below with the basi-sphenoid, and forms the lateral boundary of the occipital foramen. Above, it * Tran actions of th Zoological Soci ty, vol. iv. THE STRUCTUHE OF THE PIKE'S SI<ULL. 171 unites with its fellow for a short distance, and so constitutes the upper boundary of that foramen, shutting out the superior bone from any share in its formation. Except in this point, it will be observed that the three bones quite correspond with the ex-occipitals and supra-oc_cipital (8.0.) of Man. The ex-occipital (E. 0.), however, further differs from that of }Ian in that it is perforated and ~ot. merely notched, by the foramen for the eighth pair, and that It IS produced in front of, and external to, this foramen, so as to enter largely into the chamber which lodges the lower and posterior part of the organ of hearing. Furthermore, there is no perforation for any hypoglossal nerve, that nerve not being represented in a distinct form in the fish. Fig. 70. f, r /1,<;.?•. Ej10. Fig. 70.-The basal and lateral bones of the skull of a Pike viewed from "bove '1'1 ] d tl tl . . " . 1e sqna-mosn an 1e nee penohc bones are left unshac ded • p , the p1't Ul' ta J ·Y !1O' SSa. . In the Pike, n.o bony wall separates the membranous labynntl~ from the cavity of the skull, the periotic ossifications being all situ~ted, as they are when they first appear in Man, upon the o~ter side of the capsule of the labyrinth; and this capsule is ~till. less compl~te than that of the human footus, seeing that Its Inner wall B not even cartilaginous, but remains in the · ?ondition of m~~brane. Notwithstanding the comparatively Incomplete conditiOn of the periotic bones of the fish, however, |