232 ON THE STRUCTURE OF THE SKULL. 



with the rest of the skull ; and, on the other, in the Serpents, 

 in which, as has been already stated, the quadrate bone is 

 shifted to the end of the squamosal, and the palatine, pterygoid, 

 and maxillary bones are bound only by ligaments 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 makes its appearance which cannot, 

 at present, be identified with any bone of Fishes or of Mam- 

 mals. This is the transverse bone of Cuvier (2V), which unites 

 together the maxilla with the palatine and the pterygoid. 



Eemarkable 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 maxillary 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 sacs open 

 below and behind into the cavity of the mouth, 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 Man, 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 (N [ , Fig. 95). The arrangement of the palatine 

 bones is such that, in most Crocodilia, the vomers are com- 

 pletely excluded from the roof of the mouth. 



When a tympanic cavity exists in the branchiate Vertehrata, 

 it is little more than a diverticulum of the buccal cavity, con- 

 nected by so wide an aperture with the latter, that an Eustachian 



