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Z ON THE VERTEBRATE SKELETON. 215 
supported by the bones 19, fig. 5. For the arguments by which the olfactory 
ganglions in the cod are shown to be homologous with the olfactory ganglions 
that rest upon the cribriform plate in man, and by which the medullary cords 
or crura connecting them to the rest of the brain are shown to be homologous 
with the so-called ‘ olfactory nerves’ in the human cranium, and for the ge- 
neral homology of both as primary divisions and peduncles of the encephalon, 
the reader is referred to Dr. Desmoulins, ‘ Anatomie des Systémes nerveux 
des Animaux a Vertébres,’ 1825, 8vo. t. i. p. 169; to Mr. Solly’s excellent 
treatise ‘On the Human Brain,’ 1836, p.’78; and to my ‘Lectures on the 
Vertebrata,’ 1836, p. 184. I there adopt the expressive name applied by 
MM. Vogt and Agassiz to this most anterior of the four primary divisions 
of the brain of fishes, and apply to the peduncles of the ‘rhinencephala,’ 
which are frequently of great length in fishes, the name of ‘rhinencephalic 
crura, since they are serially homologous with the prosencephalic or cerebral 
erura; and I eall that division of the cranial cavity which specially lodges 
these crura and their lobes the ‘rhinencephalic’ chamber or compartment. 
The right appreciation of the above essential characters of the most anterior 
division of the brain and the brain-case is indispensable to the accurate pur- 
suit of the homologies of the bones 13, 14 and 15, whose development, espe- 
cially of the pair no. 14, is governed by that of the rhinencephalon. In man 
the all-predominating cerebrum, overarching the mesencephalon and epen- 
cephalon behind, and the rhinencephalon in front, so modities the surround- 
ing cranial bones as to obliterate every part of the rhinencephalic division, 
save the terminal fossa that immediately supports the so-called ‘olfactory 
ganglia,’ which fossa seems, as it were, to be unnaturally drawn in and 
blended with the great prosencephalic chamber, by reason of the enormous 
outswelling development of the proper spines or roof-bones of that chamber, 
the frontals. Still, even here, through the absence of any commissural band 
connecting tegether the rhinencephala, a fibro-membranous process of the 
endoskeleton extends between them, and into this septum ossification extends 
' from below, called the ‘crista galli. In the cod-fish the homologous parti- 
tion between the rhinencephala is cartilaginous, and it extends some way back 
between their crura, not being opposed by a coextended overhanging cere- 
brum with great transverse commissures. In many fishes (e. g. Xiphias) the 
outlet of the olfactory nerves, which notches the inner side of no. 14 in 
the cod, is converted into a foramen by the extension of ossification around 
the mesial surface of the nerves. Where the olfactory nerves are sent off 
from the ganglions in great numbers (e. g. Raia), they perforate a mem-, 
brane before reaching and ramifying upon the vascular pituitary sac. In 
man, the homologous membrane, or basis of the olfactory capsules, is ossi- 
fied, and called from its numerous apertures the cribriform plate. The holes 
which these cribriform plates fill up are homologous with the foramina, or 
grooves forming the outlets of the olfactory nerves in the bones no. 14 in fishes 
(figs. 4 and 5). 
The grounds for this homology are so plain that we cannot be surprised 
that they should have been early appreciated, as e. g. by the painstaking and 
philosophic Bojanus in 1818*. I never could comprehend the precise mean- 
ing of the statement with which Cuvier opposed his view :—“ M. Bojanus, par- 
tant sans doute du trou qu'il a dans plusieurs poissons pour le nerf olfactif, en 
fait une lame cribleuse de l’ethmoide ; mais cette opinion, qui n’a pas ce soutien 
dans toutes les especes, est réfutée d’ailleurs par les autres rapports de cet os 
avec les os voisins}+.” Cuvier seems to have thought the ground of Bojanus’s 
opinion to be cut away by the fact that in the cod and some other fishes the 
* Isis, heft iii. p. 503. t Ilistoire des Poissons, i. p. 235. 
