the basisphenoid, and the posterior surface is crescehtically concave before the
inferior border of the foramen magnum. The tympanic plate is thin rough and
slightly concave on its outer surface, and behind it is the elongated surface which
unites with the exoccipital.
In the adult skull the line of union with the basisphenoid can sometimes be
traced, but its union with the exoccipital cannot be detected. All the foregoing
processes become greatly intensified in the adult, more especially the process external
to the pituitary fossa; these project outwards beyond the cranial cavity as two
strong buttresses on which the parietals rest, although they do not unite with these
bones. Their extremities point to the internal auditory meatus, and their posterior
sides define anteriorly a strongly marked groove for the seventh pair of cranial nerves;
immediately behind this there is another prominent groove for the eighth pair,
which arch over the strong process defining the anterior wall of the precondyloid
foramen and is so closely applied to the exoccipital that this foramen is all but
perfect.
The basisphenoid (PI. XL, fig. 11).—The alisphenoids are outwardly and upwardly
curved, and their excephalic aspect much hollowed in the middle from before
backwards, but convex from side to side, with a considerable concavity at the base of
the wings for the reception of the lobes of the cerebrum. The wings may be referred
to two portions, an anterior, by far the higher and larger, and a posterior, backwardly
and upwardly projecting section, that lies on the rough surface of the basi-occipital
external to the pituitary fossa. The cerebral hollow of the anterior portion of the
wings is very thin and easily fractured, hence the imperfections in the figure (fig. 11),
but its upper extremity is very thick and rough. Immediately 'below this latter
portion there is a wide groove directed longitudinally forwards, leading to the sphenoidal
fissure and prolonged backwards as a distinct process along which the two
superior divisions of the fifth nerve pass. There is a large crescentic foramen just
below the posterior process, a backward continuation of the groove for these divisions
of the fifth nerve and through which the inferior maxillary division of the
same nerve passes outwards through the alisphenoid and hence the foramen ovale. At
the base of the posterior division of the alisphenoid there is another foramen (spinosum)
which transmits a branch of the plexus at the base of the skull. The anterior division
of the alisphenoid is applied by its front margin to the orbitosphenoid, while by
its thickened superior rough surface it articulates with the inferior thickened rough
angle of the parietal; it also rests on the external surface of the inferior posterior
external angle of the parietal, i.e., being external to that process with the frontal process
of the pterygoid external to it and excluding it from appearing in the outer wall
of the skull, although in the temporal fossa it is close behind the superior border of
the isolated portion of the pterygoid, that is wedged in between the frontal, parietal
and squamosal.1 At this portion, the wall of the cranium consists of three layers
of bone. These, from without inwards, are; first, the downward and forwardly
1 See Flower’s remarks on the structure of the temporal fossa of JP la ta n is ta as compared with th a t of
I n i f i , 1. c. p. 90.
projecting pterygoid process of the squamous which lies on the pterygoid ; next,
the pterygoid itself ; and lastly, the anterior half of the alisphenoid. The anterior
surface of the body has not united with the presphenoid in the specimen 14 inches
long* and even in adult skulls this suture is always more or less distinct. The
anterior portion of the under surface of the body is rod-like in the middle to
permit of its fitting into the deep groove of the vomer, while the sides slope upwards
and outwards and rest in the deep grooves on the posterior sphenoidal processes of
the pterygoid, which continue forwards from the depending tympanic plates to the
external margins of the internal nares ; these plates thus extend from the exoccipital
on to a line with the posterior margin of the vomer. The basisphenoid portion
contributed to these plates is thus comparatively small, not extending forwards
beyond the two posterior thirds of the bone. They are directed downwards, backwards
and outwards, and although they become firmly amalgamated with the same
processes of the basi-occipital, they are separate in the adult skull from the similar
plates of the pterygoid which overlap them.
The posterior portion of the alisphenoid rests, as already stated, on the rough
surface of the basisphenoid external to the pituitary fossa, and is perforated near
its base by what appears to be the foramen spinosum. Its upper external angle
has a marked depression on which the Gasserian ganglion lies. In young skulls
this portion of the wing is very thin, and its outward angle abuts against the
anterior margin of the internal auditory [meatus ; the periotic in youth projecting'
into the lateral wall of the skull, while the tympanic below it forms part of its
floor. In adult life, this portion of the alisphenoid is enormously thickened and
completely consolidated with the underlying process of the basi-occipital J the two
simulating a periotic in form and consistence and in their position in supporting the
Gasserian ganglion. By their after outward growth and by the inward thickening
of the base of the parietal against which they abut, the true periotic and tympanic
are entirely excluded from the inner wall of the cranium ; and the imperfection
at the base of the skull in this region, which in the young is nearly continuous
from before backwards, is divided into two distinct sections, the foramen lacerum
medium and the foramen lacerum posterius.
The upper margin of the posterior end of the body of the bone, although it
does not form any part of the pituitary fossa, is so close to its posterior margin that
it may almost be regarded as part of that border.
The orbitosphenoid.—This bone, which is quite distinct from the basisphenoid
in youth and evidently for many years afterwards,- Unites apparently with the
mesethmoid in uterine life. In the young, however, and even in the adult, the
anterior limits of the bone in the interior of the skull can be detected, but its position
undergoes a remarkable change from youth to age. In the former it constitutes
part of the floor of the cranium, being in the same axis as the basisphenoid, but in
the latter stage it is almost at right angles to that bone and forms the lower portion
of the anterior wall of the cranial cavity. During these changes, what appears to be
the anterior limit of the bone becomes more intensified than it is in youth, assuming
Q 3