192 report— 1846. 



lions of the orbitosphenoid and aiisphenoid are always less constant than 

 their interior ones. By these latter characters, and still better by their nerve- 

 outlets 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 aiisphenoid in rep- 

 tiles, are more consistent than the great French anatomist in regard to the 

 aiisphenoid of fishes. Dr. Hallmann, accepting Cuvier'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 aiisphenoid 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 -f-. 

 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 aiisphenoid 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 retrograclation of the aiisphenoid, ought to 

 prepare us to expect that nerves which traverse the petrosal in mammals 

 should perforate the aiisphenoid in reptiles and fishes. And so we find 

 in the carp that the glossopharyngeal even perforates the posterior border 

 of the aiisphenoid ; 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 aii- 

 sphenoid. I concur entirely with Cuvier and M. Agassiz in their determi- 

 nation of the aiisphenoid 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 Reptilia, 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 contention and not & 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 aiisphenoid 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 sphenoide.' 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- 



* Ossemeus Fossiles, -tto, t. v. pt. i. p. 81. 



f Der vergleidiende Ostcologie des Schlafeubeins, p. 01. 



