192 REPORT— 1846. 
tions of the orbitosphenoid and alisphenoid are always less constant than 
theirinferior ones. By these latter characters, and still better by their nerve- 
vutlets and their relations to the primary divisions of the encephalon, are 
they rightly and truly determinable. The German authors who have fol- 
lowed Cuvier in his views of the special homology of the alisphenoid in rep- 
tiles, are more consistent than the great French anatomist in regard to the 
alisphenoid of fishes. Dr. Hallmann, accepting Cuvicr’s characters of the pe- 
trosal, taken from its internal position and lodgement of the whole or part 
of the labyrinth*, naturally applies them to the alisphenoid in fishes, and 
adds to the grounds for regarding that bone as the ‘ petrosal,’ that it is in 
some fishes perforated by the opercular branch of the great trigeminal nerve+. 
But, admitting the homology of the opercular nerve with the facial nerve of 
mammals, yet its wider homology and essential character as a motor division 
of the great trigeminal nerve must not be lost sight of: its origin in close 
contiguity with the great sensory portions of the trigeminal in fishes accords 
better with the character of that nerve as the great spinal nerve of the brain, 
than it usually presents in higher classes; and it is surely no important de- 
parture of the alisphenoid from its normal character, that it should give exit 
to both motory and sensory divisions of the great nerve with which it is so 
intimately associated from man down to the fish. Indeed, the progressive 
withdrawal of the bony petrosal from the interior of the skull and the con- 
comitant backward extension, or retrogradation of the alisphenoid, ought to 
prepare us to expect that nerves which traverse the petrosal in mammals 
should perforate the alisphenoid in reptiles and fishes. And so we find 
in the carp that the glosso-pharyngeal even perforates the posterior border 
of the alisphenoid; but its origin close to the acoustic and facial nerves 
in fishes diminishes the force of the argument which might be drawn from 
this exceptional perforation, in favour of the petrosal character of the ali- 
sphenoid. I concur entirely with Cuvier and M. Agassiz in their determi- 
nation of the alisphenoid in fishes; but, if the great share which that bone 
in reptiles (figs. 9 and 10, 6) contributes to the formation of the otocrane, 
if the anterior position of the foramen ovale, and the superior connection of 
the bone with the supra-occipital, are proofs (as Cuvier believed) of its ho- 
mology with the petrosal in the class Feptilia, they ought also, as Hallmann 
and Wagner contend, to establish the same special homology of the bone (6) 
in the class Pisces. But none of these are essential characters of the petrosal. 
The petrosal is a contentum aud not a paries, or any part of the parietes of the 
cranial chamber or otocrane lodging the organ of hearing: it is the outermost 
tunic, membranous, gristly, or bony, of the labyrinth or essential part of the 
acoustic organ. Had the above-cited anatomists clearly appreciated the 
general homology of the petrosal, they could scarcely have failed to detect 
its special homologies in the vertebrate series. Cuvier was evidently guided 
to the true determination of the alisphenvid in fishes, less by its own essen- 
tial characters, than by observing in certain fishes, the perch and cod for ex- 
ample, a partial ossification of the acoustic capsule, to which, therefore, he 
assigned the name ‘rocher.’ And, having thus satisfied himself of the ex- 
istence of the homologue of the ‘pars petrosa,’ &c., he could not but assign 
to the bone which rested below upon the basisphenoid, which protected late- 
rally the optic lobes and gave exit to the third division of the trigeminal nerve, 
the name of ‘grande aile du sphénoide.’ But all these characters equally 
coexist in the bone which Cuvier calls ‘ rocher’ (petrosal) in the crocodile and 
other reptilia. He was not aware, however, that in both gavials and cro- 
codiles a distinct ossicle, the veritable homologue of the intra-cranial pyra- 
* Ossemens Fossiles, 4to, t. v. pt. i. p. 81. 
+ Der vergleichende Osteologie des Schlafenbeins, p. 64. 
