ON THE VERTEBRATE SKELETON. 195 



cessory to the formation of the floor of the ear-chamber : the three principal 

 bones are united, as in the crocodile, by a triradiate suture. The petrosal, 

 which, like the squamosal, was gradually more and more withdrawn and 

 shut out from the cranial cavity, as we decended from mammals, now entirely 

 disappears from view : and it retains its primitive cartilaginous state in ser- 

 pents as it does in chelonians, lizards and batrachians. The essential cha- 

 racters of the exoccipital (2) are manifested by its relative position and con- 

 nections ; by its affording exit for the vagal (#) and hypoglossal (Jig) nerves, 

 and by its protecting the sides of the epencephalon. The alisphenoid (e) is 

 not less clearly indicated by its constant and essential characters ; it rests below 

 upon the basisphenoid (5), it articulates above with the parietal (7), and 

 behind with the cartilaginous petrosal ; but the otocranial plate being, as in 

 the crocodile, unusually extended backwards, unites with the basioccipital 

 (1), exoccipital (2) and supraoccipital (3), in almost equal proportions, and 

 becomes directly perforated by the acoustic nerve (ac). Its chief foramen 

 (tr), however, is, as usual, that which answers to the foramen ovale in the 

 human alisphenoid, and which gives passage, as in fishes, to the great third 

 division of the fifth, and to the branch which is homologous with the 

 contribution by the fifth to the 'nervus lateralis' in many fishes, and at 

 the same time with the nerve called ' chorda tympani ' in anthropotomy. 



In the frog I have given an external view of the alisphenoid (6) and the 

 cartilaginous petrosal (16) in their undisturbed connections, in fig. 13, with 

 the surrounding bones. The alisphenoid is here perforated, as in Man, by 

 both a foramen ovale and foramen rotundum (tr.) : it forms posteriorly the 

 fore-part of the chamber for the cartilaginous petrosal, and usually coalesces 

 with the mastoid (s), which overarches the petrosal : the back wall of the 

 otocrane is contributed, as usual, by the exoccipital (2) ; the floor by the 

 homologue of the coalesced basisphenoid and basioccipital. Had the outer 

 part of the petrosal (10) been the seat of a partial ossification, a bone would 

 have resulted corresponding precisely with Cuvier's ' rocher ' in the cod and 

 perch : but the immediate capsule of the labyrinth retains the same histolo- 

 gical condition in the batrachia as it does in the carp and pike, and as in the 

 salamandroid polypterus and lepidosteus : in the latter fish, at most, the only 

 ossified part of the petrosal forms a small bony cup covering the posterior 

 extremity of the outer semicircular canal*. 



The attention of the justly celebrated ichthyotomist of Neuchatel appears 

 to have been too exclusively occupied with the persistent embryonic condi- 

 tion of the ' petrosal ' in these highly organized fishes, to gain that true and 

 clear idea of the essential nature of the petrosal of which its partial ossifica- 

 tion in the perch and cod is indicative. Adopting the opinion of Cuvier, in 

 preference to that of Meckel and Hallmann, touching the special homology 

 of the alisphenoid, M. Agassiz originally diverged into the opposite extreme 

 of repudiating altogether the existence of a petrosal in the class of fishes. 

 Thus, he says, " II devrait suffire ce me semble de voir l'organe de l'oui'e 

 presenter des modifications graduees dans toute la serie des vertebres, pour 

 se convaincre que le rocher n'existe pas du tout chez les poissons, par plus 

 que les osselets de la cavite du tympan. S'il y avait un rocher chez les 

 poissons, ce devrait etre un os qui entourerait le labyrinthe et les canaux 

 semicirculaires ; mais nous avons vu que ces parties de l'oreille interne se 

 trouvent dans la cavite du crane sans enveloppe osseuse particuliere, et pro- 

 tegees seulement par les parois des os qui entourent le rocher, la ou il existef „" 



* This condition answers to that in the human embryo of about the fourth month, in which 

 a light porous bony crust begins to be formed upon the cochlea and semicircular canals 

 commencing with the outer and upper ones, the rest of the petrosal being cartilaginous. 



f Recherches sur les Poissons Fossiles, torn. v. p. 66. 



