tympanic, there is a small zig-zag fissure immediately external to the processus
gracilis of the malleus, viz., Glasserian fissure. The articular surface for the mandible
is an ova! concavity looking forwards, inwards and downwards. The petro-
pterygo-tympanic is a comparatively thin plate arising by two roots, contracted at
its origin, but expanding at its extremity into three prongs in youngish specimens;
whereas in adult skulls the extremity is more or less rounded with a central
arched strong spine-like process, which in young skulls is the more anterior of the
three prongs. The anterior root is situated at the upper border of the zygoma at
its union with the parietal plate of the squamosal, but on its inner aspect it is continuous
with the upward ridge from the glenoid. The posterior root, which is the
largest, forms the roof of the postglenoid fossa, and lying longitudinally below
the parietal surface of the squamosal has its posterior extremity slightly nodose,
and on a line with the posterior margin of the base of the zygoma, and with a deep
hollow behind it, into which the periotic is received. The inner surface of this
pterygoperiotic plate is applied over the rough external surface of the pterygoid,
and its terminal spine arches over the orifice through which the inferior maxillary
nerve makes its exit from the cranium; being also applied to a small rough surface
on the free margin of the external orifice of the foramen ovale, dividing that
foramen, which is large, into two halves, a superior and inferior. The posterior
surface of the plate closely invests the anterior and external borders of the
periotic, and is applied more or less to the upper concave border of the long
Eustachian process of the tympanic. Its upper border is applied against the
pterygoid, which at this point overlaps the anterior division of the wing of the
basisphenoid. The ridge I have described constitutes the Tower boundary of the
very large temporal fossa, and the surface of the/ plate below it is smooth but
slightly concave.
Periotic (PI. XL, fig. 9, pe).—In position it presents two surfaces and three
borders; the former look upwards and downwards, and the latter are external,
internal, and posterior. The internal border has a deep and wide notch before the
cochlea, prolonged as a deep furrow on the under surface, thus resolving this aspect
of the bone into two well-marked portions, an anterior and posterior. On the upper
surface this notch is prolonged outwards and backwards along the anterior border of
the cochlea, so that the anterior division of the bone in this aspect is more extensive
than on the opposite face. The anterior portion is a powerful inwardly curved
hook, convex from before backwards, presenting above on its inner border an
upwardly projecting sharp conical, sometimes style-like process, that fits into a
vacuity between the posterior and superior extremities of the internal and external
plates of the pterygoid. The inner side of this process is marked by a groove which
passes downwards and backwards to a very minute foramen on the inner border of
the great notch at the anterior border of the cochlea; the commencement of a
canal that opens into the aqueduct of Eallopius near its termination and doubtless
conveys the chorda tympanic nerve. Internal to this foramen on the outer surface
of the periotic, there is another small foramen leading to a nutriment canal which
runs close to the canal of the nerve. These canals can only be traced by fracturing
the bone across at various intervals, and thus tracing them to their destinations.
Into the deep notch in front of the cochlea is received the strong process of the
tympanic before the malleus, and which is usually fractured off from the tympanic
and left attached to the periotic, .so firmly is it embraced by it. A round ossicle
frequently intervenes between the anterior end of this section of the periotic and
the petro-pterygo-tympanic process and the pterygoid, and appears to be a separated
portion of the hook-like process.
The posterior division of the periotic forms by far the larger section and is
thick and massive, being defined anteriorly, on the under surface, by the afore-men-
tioned notch and superiorly by the anterior border of the cochlea. , Viewed from
the upper surface, it is oblong, placed obliquely from without inwards and forwards
on the under surface of the skull, its inner rounded extremity being the free
border of the cochlea which projects into the hinder portion of the great chamber
at the base of the skull. The meatus auditorius intemus looks inwards and upwards
with a slightly forward direction, and its external upper margin is thin and spicular,
projecting considerably above the interval, which is also a thin raised ridge. The
margin of the orifice as a whole gives rise to a slight tube, which points towards
the opening at the base of the skull, through which the seventh pair of nerves pass
out, and which corresponds to the deep furrow on the basi-occipitai behind the
powerful posterior wing of the basisphenoid ; the periotic in the adult being entirely
outside the cranial wall, while in young skulls the cochlear portion projects
strongly into the cavity of the skull. The meatus auditorius intemus is a deep
circular pit, marked by a series of cribriform spirals with the opening of the aqueduct
of Eallopius near the bottom of the pit, in its outer wall. Immediately external
to the meatus auditorius intemus and nearly in a line with its hinder margin,
there is a prominent well-defined pit, in the bottom of which occurs the minute
aqueduct of the vestibule, the anterior wall of which usually bears a long spicule.
I t may be taken as a guide to the position of the semi-circular canals which occur in
the petrous substance of the bone immediately anterior to it ; the aqueduct of the
vestibule opening internally, anterior to the inferior end of the superior or posterior
semi-circular canal. Below this, on the posterior border of the bone, is the vertically,
somewhat oval, and large fenestra rotunda, in the bottom of which are visible
the two spiral laminæ of the modiolus and wall of the cochlea separated by a narrow
intervening fissure. Between the fenestra rotunda and the internal auditory meatus
is the short canal, the aqueduct of the cochlea. The under surface of this portion
of the posterior division of the periotic presents two powerful, fang-like processes,
the anterior of which fits into the deep pit on the postauditory process of the squamosal,
the posterior being embraced by the exoccipital and the vaginal process of
the tympanic. The deep notch between these two processes is completely occupied
by the inner and posterior walls of the postauditory pit, which so grasp and embrace
thé anterior process that the periotic can only be removed by fracturing the walls
of the pit.