‘ 
ai 
y 
. 
+ 
4 
. 
> 
%& 
=. 
3 
~ 
ON THE VERTEBRATE SKELETON. 211 
dismemberments of the human temporal bone ; and we cannot climb to the 
higher generalizations of anatomical science, except by the firm steps of true 
and assured special homologies. There are more important subjects than 
homologies, no doubt ; but nothing is more important than truth, in whatever 
path we may be in pursuit of her. 
Orbitosphenoid.—As evidence will be given in the section on ‘General 
Homology’ that both squamosal and tympanic belong to a quite distinct 
category of bones from the parts of the ‘temporal’ which have just been 
discussed. I shall proceed next to the neurapophyses that precede the 
alisphenoid. 
As the determination of this bone (c in all the figures) involves that of 
the orbitosphenoid (10), which has rarely been mistaken* for any other bone 
‘than 6, there remains little to be added in proof of its homology after 
what has been advanced respecting the alisphenoid. The most constant 
character of the orbitosphenoid is its relation to the optic nerve, which either 
perforates or notches it, whenever the ossification of the primitive cartilage 
or membrane holding the place of the bone is sufficiently advanced, which 
is not always the case in fishes, especially those with broad and depressed 
heads, and still more rarely in lacertine saurians. The recognition of the 
orbitosphenoid is also often obscured by another cause, viz. the tendency in 
the class Reptilia, and especially in ophidians and chelonians, to an extension 
of ossification downwards into the primitive membranous or cartilaginous 
neurapophysial walls of the brain-case, directly from the parietal and frontal 
bones. 
_ In the fishes with ordinary-shaped, or with high and compressed heads, 
the orbitosphenoids are usually well-developed: they are, however, repre- 
sented by descending plates of the frontal in the garpike ; and they are, like the 
alisphenoids, mere processes of the basisphenoid in the polypterus, which thus 
offers so unexpected a repetition of the human character of the correspond- 
ing partst. In the cod (fig. 5, 10) they are semielliptic, raised above the pre- 
sphenoid (9), suspended, as it were, between the alisphenoid (6) and the 
frontal (11), and bounding the sides of the interorbital outlet of the cranium: 
the optic nerves pierce the unossified cartilage closing that aperture, imme- 
diately beneath the bone itself. In the malacopterous fishes with higher 
and more compressed heads, the orbitosphenoids are more developed ; they are 
directly pierced or deeply grooved by the optic nerves, and are pierced also 
by the ‘nervi pathetici’ in the carp. The crura of the olfactory ganglions 
(rhinencephala) pass out of the interorbital aperture of the cranium by the 
upper interspace of the orbitosphenoid, into the continuation of the cranial 
cavity which grooves the under surface of the frontal, in their course between 
the orbits to the prefrontals. The orbitosphenoids protect, more or less, the 
sides of the prosencephalon ; and this function, their transmission of the optic 
nerves, their anterior position to the alisphenoids, and their articulation 
above with the frontals, establish their special homology from the-fish up to 
man. 
In certain fishes a distinct centre of ossification is set up in the median 
line of the fibrous membrane or cartilage, closing the interorbital aperture 
of the cranium, below the orbitosphenoids, and extending forwards as the in- 
terorbital septum. ‘The bone (represented in outline in fig. 5, at 9') extends 
downwards to rest upon the presphenoid (2b. 9), and bifurcates, as it ascends, 
' * Geoffroy in his memoir on the skull of birds (Ann. du Mus. x.), indicates the orbitosphe- 
noid at P, fig. 2, pl. 27, as the ‘rocher’: and Cuvier describes it as part of his ‘os en cein- 
ture’ in anourous batrachia. 
_ + Agassiz, Recherches sur les Poissons Fossiles, ii. p. 38. 
