OCR Text |
Show 168 ON THE STRUC'l'URE OF THE SKULI1. rated by a plate of cartilage, which corresponds to a certain extent with the crista galli of the human sh:ull. Fig. 69. Fig. 69.-fiide and upper views of a Pike's skn1l (after Agas. iz).-a, the articular facet for the hyomandibular bone; x , the "parasphenoitl ;'' y, the true basi-sphenoid· z, the alisphenoid. ' Immediately in front of the pituitary fossa a thin plate-lilw ossification, y, is developed in the cartilage, and this plate sends off backwards and a little upwards, upon af'h sid , a prof'css which is connected posteriorly with the cranial floor. These two processes consequently lie at the sides of the pituitary fossa, and the "Y-shaped bone," as it has been well termed, thus furnishes part of the front and side wall of that fossa. The next ossification to be uotod in the cranio-facial axis of the Pike is the great bone m (E ig . GG to G9), which stretches, like a splint, along the greater part of tho length of the base of the skull. The lower face of the hinder half of this bone is free, while that of its front half is covered by the bone, Vo. Tho upper face of its hinder half articulates, at first, with the lower surface of B. 0., but is then free for some distance, fonning the floor of the canal for the orbital muscl , and articulates by expaHdod aliform processes of its sides with the lat ral walls of that ca.nal· At the front part of the canal it exhibits the elevation which forms the floor of the pituitary fossa, and then, depressed at the sides, but exhibiting a median superior ridge, it underlies tho inter-orbital and ethmoidal cartilage . THE S'l'RUCTUHE 0})' 'l'HE PII<~'s SKULL. 169 The last ossification of the cranio-facial axis is a depressed bone Vo, thicker in front than behind, which fits on to the undersurface of the anterior half of the bone just de cribed, and extends beyond it to the front end of the snout. The under-surface of this bone is free, enters into the middle of the roof of the palate, and bears teeth. In comparing the cranio-facial axis of the Pike with that of Man, two pair of bones appear, at once, to correspond so closely that no reasonable doubt can be entertained as to their homology. These are the posterior and anterior bones of the series in each case. The former, in its relation to the spinal column, to the medulla oblongata, and to the lateral arches of the skull of the Pike, is precisely comparaLle with the basi-occipital of Man; while the anterior bone as exactly answers to the vomer of man; except that the fish, being devoid of any communica6on between the olfactory chambers and the cavity of the mouth, the vomer has a different form, and has of course no relation to nasal passages. Again, it seems obvious that the ethmoid is represented only by cartilage, as in the footal state of the human skull, for there is no ossification in that portion of the cranio-facial axis which li~s between the olfactory sacs. .A.nd the like appears to be true of the presphenoid, for all that vertical plate-like portion of the cranio-facial axis which lies between the orbits, and beneath the peduncles of the olfactory lobes, and in front of the crossing of the optic nerves, is merely cartilaginous. The Y-shaped bone forms part of the front and side walls of the pituitary fossa, and its upper prolongations are connected behind with the bones Pr. 0, and with the floor of the cranial cavity. In this floor, the long cartilaginous plate, already mentioned, constitutes the hinder boundary of the fossa, and separates the Y-shaped bone from the basi-occipital. Now, the proper basi-sphenoid (that is to say, the central ossification taken apart from the lingulm) forms the front boundary of the pituitary fossa in Man, but extends obliquely downwards in front of it as the stmn of the Y -shaped bone does in the Pike. |