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Show 166 ON '!'HE S'l'HUC'l'UHJ!} OF 'fHE SKULT1. auditory apparatuses are fixed within. their chambers, while tho eye is freely moveable within the orb1 t. . Thus, for the Pike, I n1ay repeat the phraseology which I employed in giving a general description of the skull of l\1an. It consists of an axis, of upper and lower arches, and of chamLors for the sensory organs. The next point is to ascertain how far thi correspoHdence, thus traced generally, extends into the details of the composition of the skull; and here we may conveniently begin, as before, with the study of the cranio-facial axis. Viewed as a whole, this axis is rounded and thick l;ehind, compressed from side to side in the median reo·ion, and thickened and depressed in front. It is composed, as I have said, partly of bone and partly of cartilage. Behind, it consists of a single well-ossified mass (B.O.), which offers, po teriorly, a deeply excavated conical articular facet., quite similar to that presented by the body of the first vertebra, with which it articulates. Anteriorly, it is .also excavated in the middle, its conical cavity tenninating the canal for the orbital muscles behind. Its upper face forms the hind r part of the floor of the cranial cavity and the inferior boundary of the occipital formnen. Its lower face is bevelled off in front, and articulates with the hinder part of the upper face of the bone ro, Fig. 68. Laterally and posteriorly, it articulat s with the bones (E.O.), which constitute the lateral boundaries of the occipital foramen; while, laterally and anteriorly, its deeply-excavated surface is free, and forms part of the de p chamber in which the sacculus of the auditory organ is lodged. The great r part of this bone is solidly ossified throughont., but its conical anterior cavity is lined by a thin shell of bone, which is separated by a continuous layer of cartilage, thicker above than below, from the rest of the osseous mass. In a longitudinal section (Fig. G8) of a fresh Pike's skull, the upper part of this layer of cartilage is readily s en, and can be traced without interruption, fro1n the axis of the bone under description as far forwards as the posterior margin of the pituitary fossa, and therefore, for a long distance in front of the anterior termination of the bone B. 0. The layer of cartilage THE STHUCTURE OF 'l'IIE PIKE'S SKULh 167 bends down at the sides, and so enters into the lateral walls of the cavity for the orbital muscles. The cartilage, however, does not immediately constitute the floor of the sh:ull, or the roof and side walls of the canal for the orbital muscles, seeing that it is coated over, on both its faces, by bony matter, which is con·· tinuous with that forming the inner and the outer faces of the bone Pr.O. Although there can be no doubt, then, that. the cartilaginous lamella in question forms part of the basi-cranial axis, it docs not, strictly speaking, form part of the .floor of that skull, being shut out therefron1 by the extension over it of the ossifications (Pr. 0.), towards the middle line. Leaving these ossifications out of consideration, however, it may be said that the free edge of the n1iddle part of the cartilaginous lamella forms the posterior boundary of the fossa for the pituitary body, which dips down, surrounded by _membrane, through the centre of the canal for the orbital muscles, and rests upon the concave surface of an elevation of the bone mat P, Fig. 68. Immediately in front of Fig. 68. Fig. 68.-Longitudinal and vertical section of a fresh Pike's sku1l. The cut surface ot cartilage is dotted. For S. V.C., P. V.C., read a.s.c., p.s.c., as in Figs. G6 and 67. this elevation cartilage reappears, and extends, as an interorbital, ethmoidal, and internasal septum, to the end of the snout. The cranial cavity rapidly narrows above the cartilaginous inter-orbital septum, and ends where the olfactory lohes abut against the olfactory sacs. It appears to terminate much sooner, however; for the olfactory lobes, after running parallel with one another for s01ne distance, diverge, and become sepn- |
