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Show 204 ON '£HE S'l'RUCTURE OF THE SKULL. Eustachian tubes have separate openings into the pharyngeal cavity, and curve upwards and baclnvards from the latter round the inferior and posterior edges of the quadrate bones to open into the tympana. In Birds the tympanic cavity is roofed over by the squa-mosal, while a more or less complete floor is furnished to it by the basi-sphenoid, and a back ·wall by the produced ex-occipital (and opisthotic?). It may be completed in front by fibrocartilage or even by bone, and the membrana tympani is fastened to the outer margin of these boundaries of the tympanum and not to the quadrate bone. The Eustachian passages ordinarily traverse the basi-sphenoid, and when they reach the base of the skull unite into a single, cartilaginous, common Eustachian tube, which opens in the middle line, on the roof of the mouth. Fig. 96. Fig. 96.- Vcrtical an<.l t ransverse sections of the left tympan ic cavity of Crocodilus Mpo1'· catus. A, posterior, 13, anterior segment; a, bristle passed into the small lateral Eustachian passage leading from b, the posterior tympanic passage, which opens i~to c, the common Eustachian passage ; d, a bristle thrust in to the air-passage wh1ch traverses the supra-occipital; f , bristle passed into the anterior tympanic pa sage; Ca,Ca', carotid canal; Cl, fo ssa for the extremity of the cochlea· Tm, inner division of the tympanic cavity. ' THE SKULLS OF REPTILIA AND AVES. ~35 In the Orocod·ilia the tympanic cavities and Eustachian passages are still 1nore remarkably disposed.* The tympanic cavity of Orocodilus biporcatus (Fig. 89, A; Fig. 96) is distinguishable into an inner and an outer part. The latter is bounded by the squamosal bone, above and behind, by the quadrate bone, below and in front. Into the former the supra-occipital enters, above ; the quadrate, and, to a slight extent, the basi-occipital and basi-sphenoid below. To the posterior wall of the inner division, that outward and backward prolongation of the ex-occipital, which answers to the opisthotic of the turtle, contributes, while the front wall is formed partly by the quadrate and partly by the pro-otic bones. Externally the tympanum opens by the external auditory meatus-its internal wall is formed chiefly by the pro-otic, opisthotic, and epiotic. The two latter are, as I have already stated, auchylosed, respectively, with the ex-occipital and the supra-occipital. Each tympanic cavity communicates with its fellow of the opposite side, superiorly, by a wide passage, which perforates the supra-occipital bone and has a secondary diverticulum traversing the ex-occipital. Below, the tympana communicate with one another indirectly, by means of the common median Eustachian tube, the aperture of which, formed, half by the basisphenoid and half by the basi-occipital, is seen on the base of the skull behind the posterior nares (c). Each tympanum communicates with the common Eustachian tube by two passages: one, wide, from the posterior and inferior part of the tympanum (b) ; and one, very narrow, from its anterior and inferior region (f). The two exits are separated by that part of the floor of the tympanum which is form.ed by the basi-sphenoid and basioccipital. This presents, behind, a hemispherical fossa (Ol) into which both the basi-sphenoid and basi-occipital enter, and, in front, a round aperture with raised edges, situated altogether in the basi-sphenoid ( oa). The fossa lodges the distal blind end of the cochlea. The aperture leads into a canal, which, passing * These were first carefully described by Professor Owen in a memoir published in the Philosophical Transactions for 1850. Windischmann, " De P enitiori auris in Amphibiis Structura" (1831;, has given but a very imperfect account of them. |
