ON THE VERTEBRATE SKELETON. 213 



panding, to where the frontals and the modified prefrontals (ethmoid) form 

 the actual anterior boundary wall of the cranial cavity ; the chief distinc- 

 tion between the condition of this boundary in the mammal and the fish, 

 being, that whereas it is perforated by numerous apertures in the mammal, 

 the olfactory nerves in the fish escape each by a single foramen or groove 

 in the homologous bones. As beautiful as true was that clear perception 

 by Bojanus of the homology of the simply perforated prefrontal of the fish, 

 with its sieve- like homologue in the class in which the olfactory sense reaches 

 its maximum of development and activity, and modifies all around it. The 

 coalesced bases of the orbitosphenoids, forming the anterior boundary of the 

 bed of the optic chiasma, answer to the separate ossification called ' eth- 

 moi'de cranien' by Agassiz, in fishes : it has the same relation with that con- 

 tracted area of the cranium answering to the interorbital aperture of the cra- 

 nium in fishes, which the so-called cranial ethmoid (entosphenoid) presents 

 in fishes ; and this same entosphenoid (fig. 5, 9') has as little relation to the 

 formation of the canals pierced by the olfactory nerves in fishes, as the 

 orbitosphenoid has in mammals. The olfactory, rhinencephalic or anterior 

 division of the cranial cavity in most fishes has its lateral bony walls incom- 

 plete, and it opens freely, in the dry skull, into the large orbital chambers 

 below, which are then said to have no septum : we see a similar want of de- 

 finition of the cranial cavity in relation to the great acoustic chambers in most 

 fishes. But in mammals the orbits are always excluded from the rhinence- 

 phalic, or olfactory compartment of the cranium* ; and a dike exclusion 

 obtains in some of the highly organized ganoid fishes and in the plagiostomes. 

 As the prosencephalic parts of the brain progressively predominate, and the 

 rhinencephalic parts diminish, in the higher mammals, the compartment of 

 the cranium appropriated to the latter loses its individuality, and becomes 

 more and more blended with the general cavity. In the elaborate ' Icono- 

 graphy of Human Anatomy' by Jules Cloquet, for examplef, the small pe- 

 culiarities of the 'trou borgne' and the ' apophyse crista galli' are both in- 

 dicated, and very properly ; but the rhinencephalic or olfactory division of 

 the cranial cavity, though defined by the suture between the orbitosphe- 

 noids and prefrontals and lodging the olfactory ganglia or rhinencephala, — 

 so important an evidence of the unity of organization manifested in man's 

 frame and traceable in characters, strengthening as we descend to the lowest 

 osseous fishes — is wholly unnoticed. Thus, very minute scrutiny, con- 

 ducted with great acuteness of perception of individual features, qualities 

 highly characteristic of the anthropotomists of the school of Cloquet, being 

 directed from an insulated point of view, prove inadequate to the apprecia- 

 tion of sometimes the most constant and important features of their exclusive 

 subject. 



But to return to the homology Pip. jg^ 



of the orbitosphenoids. In the me- 



nopome these neurapophyses are t^ a mt3 ^=-~- gF ----=^-.- -iT, 



elongated parallelograms, perfo- 

 rated by the optic nerves, and are 

 distinct bones. In the great bull- 

 frog (liana boans) they present a 

 similar form (fig. 13, 10), but are 



Confluent with the prefrontals (14) : side ^ew of cranium (Rana boons), nat. size. 



in both batrachians an unossified sPace intervenes between them and the ali- 



* This is not to be confounded with the olfactory chamber itself, lodging the organ of 

 smell, 

 f Manuel d'Anatomie Descriptive, 4to, Atlas, pi. 8, fig. 2. 



