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Show ~32 ON THE STRUCTUR-E OF THE SKUL!J. with the rest of the skull; and, on the other, in the Serpents, in which, as has been already stated, the qu~drate bone is shifted to the end of the squamosal, and the palatine, pterygoid, and maxillary bones are bo?nd only by liga~ents to the skull, so that the utmost possible amount of play IS allowed to the bones which surround the mouth. In many Reptiles a bone n1akes its appearance which cannot, at present, be identified with any bone of Fishes or of Mammals. This is the transverse bone of Cuvier (Tr), which unites together the maxilla with the palatine and the pterygoid. Remarkable differences are noticeable in the degree to which the premaxilla is developed in the various orders of Reptiles and in Birds. In the Snakes it is very small, or rudimentary; in the Lacertilia, Chelonia, and Crocodilia it has a moderate size· while in the extinct Ichthyosauria, and still more in Birds, ' . the premaxilla attains vast dimensions, completely surpassing the 1naxillary element, which in Birds is reduced to a mere bar of bone, connected by similar slender rods, which represent the jugal and quadrato~jugal, with the outer part of the distal end of the quadrate bone. In the Ophidia, most Lacertilia, and Birds, the nasal saes open below and behind into the cavity of the n1outh, by apertures placed between the vomer and palatine bones, which correspond with what I have termed the "median nares" in Man ; or there is, at most, an indication of a separation between the oral cavity and the nasal passage, produced by the sending downwards and inwards of a process by the maxillary and palatine bones on each side. But, in the Crocodilia (Fig. 95, B), not only the maxillary and palatine bones, as in J\rian, but the pterygoid bones, in addition, send such prolongations downwards and inwards; and these, meeting in the median line, shut off from the cavity of the mouth a nasal passage, which opens into the fauces by the posterior nares (N1, Fig. 95). The arrangement of the palatine bones is such that, in most Crocodilia, the vomers are completely excluded from the roof of the mouth. When a tympanic cavity exists in the branchiate Vertebrata, it is little more than a diverticulum of the buccal cavity, connected by so wide an aperture with the latter, that an Eustachian 'l'HE SKULJ_JS OF REP'l'ILIA AND AVES. 233 tube can hardly be said to exist. In the Ophidia and in certain Lacertilia the tympanic cavities and Eustachian tubes are alto· gether absent, and, even in the higher Lacertilia, the tympanu1n can hardly be said to have definite bony walls. In the Chelonia, Fig. 95. A B c Fig. 95.-~iews of one half of the palatine s~rface of the skull in (A) an Ostrich. (B) a Croco~1le, (~) a Python.. N, the postenor nasal aperture (median nares of Man) in the BmL fhe dotted hue traverses the posterior nasal aperture, situated between the palatine, the ~omer, ~nd the maxillary. The corresponding opening is placed between Vo and Pl 111 the Snake. NI, the posterior nasal aperture, or posterior nart:>s, of the Crocodile. on the other hand, the opisthotic and tbe pro-otic bones are produced outwards so as to form the anterior and posterior boundaries of a cavity, the antivestibulum Bojani-which is bounded externally by the great quadrate bone. ~rhe latter is funnel-shaped, and deeply notched posteriorly and inferiorly. The tympanic mernbrane is fixed to the margins of the funnel, and the so-called "columella," which answers to the stapes, is fastened by one end to this tympanic membrane, and traversing the notch and entering the antivestibulum, passes to its other insertion into the 1nernbrane of the Jenest1-·a ovalis. 'l'he |
