the character of a pronounced sutural line. The bone Is referable to two portions,
a body and wings; the latter extend quite as far on to the side of the skull as do
the alisphenoids. The wing-like portion presents two divisions, a posterior and
anterior; the former constituting the front border of the sphenoidal fissure in the
cavity of the skull, and the latter, above it, projecting into the base of the posterior
angle of the frontal. There is apparently no mesethmoid spine, as that portion of
the is let into the anterior end of the body of the orbitosphenoid. The
ridge of the falx-eerebri can be traced on to the middle of the body in some young
skulls whereas in adults the body is either concave or pitted irregularly. Halfway
between the extremity of the anterior division of the wing and the body of
'the hone, in what appears to he its mesethmoid border, there is usually, m young
skulls a considerable foramen for the passage of blood vessels; this may be either
round or transversely elongated, hut in adult skulls it becomes almost entirely
obliterated. , . , .
The most remarkable feature, however, of the orbitosphenoid is the circumstance
that it is traversed halfway between its anterior and posterior borders, and »bout
halfway between its mesial line and the outer extremity of its wings, by an
extremely small optic foramen; this is so minute and so like to the sentient foramina
that occur scattered over the inner wall of the skull, that it is very apt
to be passed over as one of tiiese. Bach optic foramen (PI. XL, fig. *; op)
is overhung by a fine bony plate, and a groove passes inwards and backwards
in young skulls, but in adult crania it is all but obliterated. In youth the foramina
are much more symmetrical in their position than in advanced life, and they always
occur in the place I have assigned them; hut in the latter périod they are generally
close to the anterior border of the basisphenoid and frequently at difierent elevations,
one being usually higher than the other; in some skulls the left is higher than the
right, and in others the right is on the higher level. The foramina aré so minute
that they only admit a fine bristle, and in adult skulls it is diffieult to pass even this
through the whole length of the canal, which is of considerable extent. A hair that
I inserted through the optic foramen of one skull again entered the interior of the
skull by the sphenoidal fissure; hut the cause of this will be apparent when the
course of the optic canal through the frontal bone is considered.
The posterior division of the wing of the orbitosphenoid articulates by its
extremity with the frontal anteriorly, and by its outer and under surface (which is,
broadly and deeply grooved for the branches of the fifth nerve passing out by the
sphenoidal fissure) with the outer extremity of the alisphenoid; this is overlapped by
the pterygoid and by the under surface of the base of the wings with the internal
superior plate of the pterygoid; the body of the bone being supported by the vomer
(consult PL XL, fig. 1, &c.)
Mesethmoid.—I t is impossible, even in young skulls, to define the exact extent
of this bone, as it unites in uterine life with the frontals (PL XL, fig. 1, me) ; but
what appear to be its limits are best seen in the internal and nasal aspect of the
cranium. I t forms the upper portion of the posterior wall of the external nares
below the level of their upper border, the external margin of which is formed by
the premaxillse. The anterior surface is divided in two by the low vertical ridge
of the mesethmoidal cartilage, which is prolonged forwards and slightly downwards
as a laterally compressed plate, received below into the groove of the vomer.
In young skulls, the anterior surface of the mesethmoid is slightly concave, with
the ridge that traverses it incompletely ossified; and it is perforated by two larger
foramina for blood vessels, one on either of its halves, with generally two smaller
foramina in its immediate neighbourhood. In adult life, the anterior surface has
the appearance as if the wings of the vomer were upwardly expanded, becoming
continuous superiorly with the upper portion of its vertical ridge; but it is open
anteriorly for the downwardly thickened portion of the central plate, from which
the septal cartilage springs, and is continued forwards. The wings of the vomer
also appear, with age, to expand upwardly and outwardly over the foramina, and
almost to invest them. The vertical ridge is thus strongly prominent in adult life,
and the perforated sides of the bone are much more regular and concave than
in youth.
On the internal surface of the skull, what appears to be the mesethmoid area
undergoes remarkable changes from youth to age. In the former period, the internal
frontal crest terminates in a vertical, rather long thick rounded ridge, which is
even prolonged in some skulls on to the body of the orbitosphenoid. At the lower
end of this ridge, there is a depression on either side of it, while immediately above,
there are three small foramina, of which one is larger and more prominent than the
others. In the adult skull, in which the orbitosphenoid and mesethmoidal areas are
completely vertical, the two feeble depressions of the young skull are converted into
deep pits, nearly half an inch in depth, separated from each other by a vertical plate,
situated rather deeply in the common opening to the two pits; its upper half being
thick, whilst its lower half forms a thin vertical plate: the bases of the pits are
perforated by foramina. In the interior of these two pits we have thus the equivalent
of the ethmoidal spine and cribriform plate. In such skulls, the internal frontal
crest takes a curve to the right side, but, notwithstanding, the falx-cerebri is
attached to the upper thickened half of the vertical plate between the two pits.
On the lower extremity of the outer side of the mesethmoid anteriorly, the palatine
bone lies between it and the frontal, the mesethmoid plate forming the upper
posterior wall of the external nares.
Pterygoids (Pl. XL, figs. 13 and 14).—The true nature of these bones was
first pointed out by Eschricht,1 in his “ Memoir on Platmista ” ; they form by far the
greater pairt of the palate. The anterior half of the external plate is convex and the
posterior half concave, especially near its hinder border. In the anterior two-thirds
of the palatal surface, the bones of either side, in the fully adult skull, are separated
from each other by the interposed vertical plates of the maxillaries, but behind
this they are closely applied. The lateral concavity of their anterior halves confers
i Trans. Roy. Acad. Sci., Copenhagen, ii, 1851. Translated by Wallich, Ann. and Mag., Nat. Hist., 1852.