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Show 240 ON THE STRuCTURE OF THE SKULIJ. The two lateral trabeculre-parts which I have also seen in Frogs, Lizards, Birds, and l\1ammals-chondrify at the beginning of the third period. At :first, they pass: distinct from one another throughout their whole length, as far as the frontal wall, on entering which they come into contact; they are more separate posteriorly than anteriorly, and they present, in their relative position and form, some sin1ilarity with the sides of a lyre. But as the eyes increase, become rounder, and project, opposite the middle of the trabeculoo, downwards towards the oral cavity, the latter are more and more pressed together, so that, eyen in the third period, they come to be almost parallel for the greater part of their length. Anteriorly, where they were already, at an earlier period, nearest to one another, they are also pressed togetl1er by the olfactory organs (which have developed at their sides to a considerable size), to such a degree, that they come into contact for a great distance and then completely coalesce; they are now most rmnote posteriorly, where the pituitary body has passed between then1, * so that they seem still to embrace it. Anteriorly, between the most anterior regions of the two nasal cavities, they diverge from their coalesced part, as two very short, thin, processes or cornua, dil:ected upwards, and si1nply bent outwards. It has been seen above that the menian trabecula does not chondrify, but eventually disappears; in its place, a truly cartilaginous short thick band grows into the fold of dura mater from the cartilaginous basal plate. Where the pituitary gland lies, there remains between the lateral trabeculoo of the skull a considerable gap, which is only closed by the mucous membrane of the mouth and the dura mater. But there arises in front of this gap, between the two trabeculre, as far as the point where they have already coalesced, a very narrow, moderately thick, and anteriorly pointed streak of blastema, which, shortly before the end of the third period, acquires a cartilaginous character and subsequent! y becomes the body of the presphenoid. :~~ The pituitary body, however~ as Rathke has since admitted, does not pass between the trabeculre, and is developed in quite a different manner from that supposed in the memoir on Coluber. THE BKULLS OF REPTILIA AND AVE~t 241 Altogether anteriorly, however, where the two trabeculre have coalesced, there grows out of this part, from the two cornua in which it ends, a pair of very delicate cartilaginous plates. At the end of the third period both plates acquire a not inconsiderable size, take the form oftwo irregularly-formed triangles, and are moderately convex above, concave below, so as to be, on the whole, .shell-shaped. The nasal bones are developed upon these, while below them are the nasal cavities and the nasal glands with their bony capsules. ' . The aloo, or lateral parts, o~ ~he two sphenoids do not grow, hke the lateral parts of the occipital bone, out of the basis cranii the foundation of which is formed by the cephalic part of th; ?horda, bu~ are formed separately from it, although close to it, In the, until then, membranous part of the walls of the cranium. The aloo of the presphenoid ( orbito-sphenoids), which are observable not very long before the termination of the third period, a~pear as two truly cartilaginous (though they never redden), Irregular, oblong, plates of moderate thickness; lie in front of the optic foramina, at the sides of the lateral trabeculre of the skull; : ascend from them upwards and outwards, and are somewhat convex on the side turned to the brain, somewhat concave on the other. The aloo magnoo ( alisphenoids) are perceptible a little earlier than these. They are formed between the eye and the ear, and also originally consist of a colourless cartilaginous substance : they appear, at the end of the third period, as irregular four-sided plates; lie at both sides of the anterior half of the ir~vesting plate of the chorda; ascend ~ess abruptly than the aloo orbitales, and are externally convex, Internally concave. . The upper p~sterio:F angle of each elongates, very early, Into a process, whwh grows for a certain distance backwards along the upper edge of the auditory capsule, and applies itself closely thereto. . The auditory capsules, or the future petrous bones, chondrify, as It would appear, the earliest of all parts of the skull: the fenestra ovalis arises in them by resorption. The ossification of the Snake's skull commences in the basi-occipital, or, at any rate, this is one of the first parts to R |
