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Show 236 ON THE STRUCTURE OF THE SKULI,, downwards and forwards in the basi-sphenoid, opens into the pituitary fossa, and lodges the internal carotid. The upper aperture of the anterior, narrow, passage from the tympanum (f) is situated in fl:ont .of the lip of the carotid canal, and, at first, lies between the basi-sphenoid and the prootic; but, soon turning inwards, it enters the basi-sphenoid, and passes beneath the carotid canal, to open into a much wider median channel. The latter ends blindly in front and above, behind and below the pituitary fossa; but, inferiorly, it traverses the substance of the basi-sphenoid, to open into the upper and front part of the common Eustachian tube (Fig. 96, B). The posterjor, wide, passage (b) leads downwards and inwards through the substance of the basi-occipital, and the two passages of the opposite tympana unite to form a short median canal, which opens, on the front face of the basi-occipital, into the common Eustachian tube (Fig. 96, A). The posterior tympanic passage Las, however, another means of communication with the exterior ; for, just before it joins with its fellow, it gives off, downwards, a narrow canal, which traverses the basi-occipital, and opens on its inferior face to the outer side of, and a little behind, the aperture of the common Eustachian tube (a, Fig. 96, A). There might, then, be said to be three Eustachian tubes in the Crocodile,-two small and lateral, one for each tympanum, and one large and median, common to both tympana. Where the posterior tympanic passage passes into the tympanum, the ex-occipital presents a round aperture with raised edges, which is the anterior termination of the posterior division of the canal for the internal carotid (0"', Fig. 96, B). In the interval between this aperture of entrance and that of exit already described, the internal carotid is unprotected by bone, and is closely adherent to the outer surface of the cochlea; which, held by the cochlear hook already described, rests inferiorly upon the fossa afforded by the basi-sphenoid and basi-occipital (Ol). The posterior wall of the tympanum also exhibits, internally, the aperture by which the ejghth pair of nerves reaches the exterior; externally, those by which the portio dura leaves, and the external carotid enters; superiorly, between the supra- THE SKULl.~S OF REPTILIA AND AVES. 237 occipital and the squamosal, is a cleft which leads.to the occipital surface in the dry skull. The early stages of the development of the skull of a Bird have already been described. The process of formation of the Reptilian skull has been admirably worked out by Rathke in the Comn:wn Snake, Ooluber natrix, and I conclude this Lecture by an abstract of his researches on this subject.* The differences between the basis of the skull and the vertebral colun1n in the earliest embryonic condition are,- 1. That round that part of the notochord which belongs to the head, more of the blastema, that is to be applied, in the spinal column, to the formation of the vertebrm and their different ligaments, is aggregated than around the rest of its extent, and- 2. That this mass grows out beyond the notochord to form the cranial trabeculm. The lateral trabeculre, at their first appearance, formed two narrow and not very thick bands, which consisted of the same gelatinous substance as that which constituted the whole invest· ment of the notochord, and were not sharply defined from the substance which lay between them and at their sides, but seemed only to be two thickened and somewhat more solid, or denser, parts of that half of the basis of the cranium, which lies under the anterior cerebral vesicle. Posteriorly, at their origin, they were separated by only a small interval, equivalent to the breadth of the median trabecula, and thence swept in an arch to about the middle of their length, separating as they passed forwards ; afterwards they converged, so that, at their extremities, they were separated by a very small space, or even came into contact. Altogether they formed, as it were, two horns, into which the investing mass of the notochord was continued forwards. The elongated space between them, moderately wide in the middle, was occupied by a layer of softer formative substance, which was very thin posteriorly, but somewhat thicker anteriorly. Upon this layer rested the infundibulum ; and in front of it, partly on this layer, partly on the trabeculro, that division of the brain whence the optic nerves * Entwickelungsgeschichte der Natter, 1839. |
