306 
DE. OLELAIO) on the KELATIONS 
Owen who considers that the central plate of the ethmoid represents m a coalesced 
condition the prefrontals of the fish or reptile*, and plays the part which we aUot 
to the lateral masses in the neural arch of the vomerine segment It cannot do 
so for these reasons: 1st, that while the lateral masses of the ethmoid are com 
tinuous with the vomer, the central plate is never truly continuous, but only con- 
tio-uous to the vomer ; 2ndly, that the central plate must, for reasons abme stated, 
pfay the part of a centrum ; and Srdly, that, as Professor Goodsie has shown, Pro= 
fessor Owen’s view is inconsistent with the relation of the olfactory nerves to the central 
3. The frontal and the central plate of the ethmoid belong to one segment. The fact 
of the central plate of the ethmoid having no early connexion with the vomer, as well as 
its tendency to remain distinct even after the vomer and it have come in contact, shows 
that it is not part of the same centrum as the vomer; therefore, inasmuch as we have 
already concluded that it is a centrum, it can form no part of the vomerine segment. 
On the other hand, the presphenoidal centrum is complete without it. In these circum- 
stances it becomes apparent that the central plate of the ethmoid is the centrum o a 
seo-ment intervening between the vomerine and presphenoidal. This opinion is strengt - 
ened by om remembering that in the Sheep the central plate of the ethmoid is ancEy- 
losed to the centrum of the presphenoid before uniting with the lateral masses, and that 
in the Hedgehog it appears on the base of the skull for a considerable distance between 
the presphenoid and vomen The frontal forms the neural arch belonging to this cen- 
trum, and is in constant connexion with it. But if, as above proved, that margm ot the 
os planum of the human subject which lies superiorly is morphologically posterior, then 
the margin of the frontal which articulates with it, viz. the inner edge of the orbitaJ 
plate, is morphologically anterior; and therefore not only the foramen ceecum, as Pro- 
fessor Goodsie believes $, but also the space occupied by the cribriform lamma, nes 
within the arch formed by the frontal and the central plate of the ethmoid. Thus the 
cribriform lamina, which in point of development is a mere lateral expansion of t e 
central plate, forms a screen across the entrance into the cavity of the ethmovomenne 
arch in the plane of segmentation, and has no further morphological importance than 
may be supposed to attach to the tentorium cerebelli. By the turning up of its anterior 
extremity to touch the frontals and nasals, the central plate of the ethmoid divides the 
neural arch of the segment to which it belongs, as well as those in front of it, into a 
* OWEX ‘ On the Archetype and Homologies of the Vertebrate Skeleton,’ pp. 131 & 135. 
t Op cit p 149 The strength of Professor Goodsib’s argument rests in this : that, according to Pro- 
fessor OwEx’s theory, the olfactoty nerves in the mammal are made to lie outside a neural arch, through 
which they pass in the reptile and fish ; which involves the supposition that the points of egress of the Olfac- 
tory nerves have been moved in the mammal one segment backwards. According to the theory advanced m 
this communication, although no doubt the bone corresponding to the mammalian central plate of the 
ethmoid passes upwards on the outside of the olfactory nerves, and does not rise up between them, ye 
both fishes and mammals the olfactory nerves are contained within the neural arch of the segmen . 
Op. cit. p. 142. 
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