Rw ee ee 
eo 1. 
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 distine- 
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- 
moide 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 eranial ethmoid (entosphenoid) presents 
in fishes ; aud 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 manimals the orbits are always excluded from the rhinence- 
phalic, or olfactory compartment of the cranium* ; and a like 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 example+, 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 Fig. 13. 
of the orbitosphenoids. In the me- 
nopome these neurapophyses are 
elongated parallelograms, perfo- 
rated by the optic nerves, and are 
distinct bones. In the great bull- 
frog (Rana boans) they present a 
similar form (fig. 13, 10), but are ee 
confluent with the prefrontals (14): Side view of cranium (Rana oans), 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. 
T Manuel d’Anatomie Déscriptive, 4to, Atlas, pl. 8, fig. 2. 
