a greater expansion, on their palatal surfaces than occurs at their middle, which is
contracted by the downward and forward prolongation of the concavity that occurs
on their posterior halves. Behind the middle, the external plates are divergent, with
their posterior inferior angles rounded off into the nasal surfaces of their internal
plates; the inferior margins of which are folded forwards in an involute manner.
Erom the central palatal contraction, backwards to the nasal plates, the two hones of
either side form a mesial long anteriorly pointed triangular palatal surface, consisting
of a fine network connecting the external and internal plates; their anterior
extremities being also closed below by a similar structure. There are considerable
imperfections of ossification also in the nasal plates. The opening into the sinus of
the pterygoids exists internal and anterior to the posterior margin of the external
plate within the posterior nares, and external to its outer wall, where that is formed
by the internal plate. The orifice, in position, is oblong and is opposite and continuous
with the anterior termination of the longitudinal fissure, along which the Eustachian
tube runs backwards to the great cavity around the tympano-periotic. In
looking through the orifice, it is seen to tend directly forward into the cavity that
exists between the two plates of the maxilla; a goose quill passed through it shows
that it is continuous with the large oval orifice and smaller foramina that exist on the
anterior surface of the maxilla, external to the premaxilla and below the level of
the external nares ; some of which foramina transmit the infra-orbital nerve. The
upper extremity of the anterior margin of the external plate is deeply notched, this
notch with the posterior extremity of the superior maxillary constituting the ptery-
gomaxillary fissure, which is continued below the sphenomaxillary fossa internally,
and anteriorly terminating in three or four oval infra-orbitai foramina. From
the upper border of the pterygomaxillary fissure, the ridge dividing the temporal
and zygomatic fossse is prolonged backwards to the foramen ovale. The superior
posterior half of the internal plate is applied over the alisphenoid, which is only
grooved externally by the inferior maxillary; but a corresponding groove on the
former completes the canal in which the nerve lies. The perfect overlapping of the
alisphenoid by the pterygoid excludes the former from the outer wall of the
cranium. I t articulates with the frontal, parietal and squamous; the latter and its
downwardly projecting pterygotympanic process, on the other hand, so overlapping
this portion of the pterygoid that only a very small portion of it appears above the
foramen ovale internal and anterior to the base of the zygoma. The superior
anterior half of the internal plate of the pterygoid is also applied to the under
surface of the orbitosphenoid. I t completes the sphenoidal fissure and confluent
foramen rotundum, and, by a thickened posteriorly concave sphenoidal process, overlaps
the free auditory plates of the basisphenoid. A thin plate internal and anterior
to this is applied to the under surface of the basisphenoid, its margin being wedged
in between the expanded base of the vomer and the orbitosphenoid, thus constituting
the roof and external wall of the internal nares. From the anterior
extremity of the orbitosphenoidal portion of the inner plate, at the posterior margin
of the pterygomaxillary fissure and immediately above the palatal, there are two
plates as thin as tissue paper directed forwards and upwards, and enclosing a wide
orifice that is downwardly and backwardly continuous with the posterior opening
into the pterygoid sinus. These two plates are wedged in between the frontal,
maxillary and palatal; the greater portion of the internal surface of the inner
plate being applied to the palatal; its free, anterior margin forms the upper
boundary of the large round infra-orbital canal of the maxillary and part of the
roof of the pterygomaxillary fissure, and constitutes its posterior wall. The
outer plate is opposed to the maxillary surface of the temporal plate of the frontal,
where it forms the anterior superior boundary of the pterygomaxillary fissure.
The alisphenoidal and orbitosphenoidal portions of the external plate project inwards
and downwards over the nasal surface of the internal plate, which forms the greater
part of the outer wall of the lower two-thirds of the nasal canals. From the
anterior point of union of the internal and external plates, a long outwardly curved
narrow thin spicular plate (op., figs. 18 and 14, PI. XL), grooved on its inner
surface, is projected forwards and appears in the inner wall of the apex of the
orbital cavity. I t is at first internal to and below the tract on the frontal traversed
by the nerves and vessels of the orbit, and afterwards forwards and outwards to
assist with the maxillary in forming its inner and upper wall. In adult skulls, this
process appears as a fragile spiculum in the upper wall of the orbit between the
frontal and maxillary.
The posterior half of the internal plate of the pterygoid, anterior to its nasal
surface, is applied to the outer surface of the lower portion of the palatine and by
the anterior half to the sides of the vomer, and to the vertical plate of the maxillary.
The anterior opening between it and the external plate is opposed to the posterior
orifice of the sinus between the two infra-orbital plates of the maxilla, and is continuous
with the imperfection of ossification that exists on the anterior surface of
the maxilla, below the infra-orbital foramina. The pterygoid completely excludes
the palatine from the palate, as first pointed out by Eschricht.
JParietal (PI. XL, figs. 6 and 7).—The parietal when removed from the
skull is more or less quadrangular, but in position it is separated above between the
exoccipital, supra-occipital and frontal; narrowing to a point between the squamosal
and the frontal, where its lower extremity is in contact with the pterygoid, which is
applied over it.. Its anterior bevelled border is applied over the posterior border of
the temporal; the upper extremity of the margin of the bone presenting a thickened
portion where it is wedged in between the external angle of the supra-occipital
margin of the frontal and the supra-occipital. I have not been able to detect any
interparietal. - The supra-occipital border is directed downwards and backwards*
In youth this margin simply overlaps the supra-occipital bending inwards, but in
the adult skull it becomes much intensified and is bent outwards. The
ex-occipital surface is short and overlaps the exoccipital, and anterior to it is the
rough surface to which the squamosal is applied. The outer surface of the
antero-inferior angle of the bone presents a smooth pit and a small sharp edge
for articulation with the upper thickened border of the anterior division of the wings of