_a_, squamous portion; _b_, petrous portion; _c_, _c′_, tympanic
portion (_c_, entotympanic; _c′_, ectotympanic); _d_, zygomatic
process; _e_, mastoid portion of the petrous; _f_, mandibular fossa;
_g_, postmandibular process; _h_, tuberculum articulare; _i_, external
auditory meatus; _j_, stylomastoid foramen; _k_, pit for tympanohyal
bone; _l_, mastoid process; _m_, grooves bounding the jugular foramen;
_n_, internal auditory meatus; _o_, appendicular fossa; _p_, hiatus
facialis; _q_, styliform process of tympanic bulla.]
The =squamous portion= (_a_) (=squama temporalis=) is thin and oval or
has the form of an equilateral triangle with rounded angles, with a
curved process, the =zygomatic process= (_d_), arising from its ventral
border. Its outer surface (Fig. 22) is convex and smooth and gives
origin to part of the temporal muscle. Its inner surface (Fig. 23) is
concave and smooth except near the margins, where it is bevelled and
rough. The roughened border is broader dorsad and caudad. The ventral
margin of the bone is turned mediad at its cranial end so that the
lateral face of the inverted portion looks ventrad. By this portion of
its lateral face the squamous rests upon the tympanic bulla, and its
edge articulates with the tentorium and the wing of the sphenoid. The
remaining (caudal) portion of the ventral border overlies the mastoid
portion (_e_) of the petrous. By the remainder of its border the bone
articulates with the parietal dorsad and with the wing of the sphenoid
craniad. The roughened portion of its inner surface overlies the margins
of both these bones.
The zygomatic process (_d_) is formed by the confluence of two roots.
One of these starts from the ventral end of the lambdoidal ridge and
passes along the ventral margin of the squama dorsad of the external
auditory meatus. The other arises abruptly from the cranioventral angle
of the bone. The process thus formed is at first broad and passes
horizontally laterad and slightly craniad. It soon grows more slender
and turns gradually craniad, while at the same time it twists so that
the surface which is dorsal at the base becomes medial at the tip; the
posterior root which is continuous with the caudal border at the base is
continuous with the dorsal border at the apex. On the ventral surface of
the base is a transversely elongated concave articular surface, the
=mandibular fossa= (_f_), for the condyloid process of the lower jaw.
Caudad of this is a sharp transverse ridge, the =postmandibular= or
=postglenoid process= (_g_), and craniad of the lateral end of the fossa
a slight tubercle, the =tuberculum articulare= (_h_). Near its apex the
zygomatic process is more slender and its ventral border is bevelled for
articulation with the malar or zygomatic bone.
The =tympanic= (Figs. 22 and 23, _c_; Fig. 24) is expanded into a large
hollow olive-shaped bone which is known as the =auditory bulla= and
encloses the tympanic cavity. Its substance is very compact. Unlike the
tympanic of most other mammals it is developed from two bones, known as
the =ectotympanic= (Fig. 22, _c′_) and =entotympanic= (_c_). These are
strongly marked in young kittens, and can usually be easily
distinguished in adult cats. The entotympanic (Figs. 22 and 23, _c_)
forms the larger part of the bulla, constituting its ventral and medial
surfaces; it is thin, smooth, and transparent. The ectotympanic (Fig.
22, _c′_) surrounds the external auditory meatus: it is thicker and more
opaque than the entotympanic. The bulla lies ventrad of the squamous,
and in an external view conceals a large part of the petrous.
On its lateral surface it presents near the dorsal border an irregular
oval opening, that of the =external auditory meatus= (Fig. 22, _i_),
which leads into the tympanic cavity. Caudad of the external auditory
meatus is a nearly dorsoventral groove, which, when the bone is
articulated, forms a part of the boundary of the stylomastoid foramen
(Fig. 22, _j_); just ventrad of this groove is a pit (_k_) which lodges
the tip of the tympanohyal bone.
Craniad the bone is produced into a short spine, the =styliform= process
(_q_), which lies in a horizontal groove in the ventral surface of the
basisphenoid. Laterad of this spine is a groove for the tuba auditiva or
Eustachian tube.
[Illustration: FIG. 24.--TYMPANIC BULLA, ISOLATED, MEDIAL SURFACE.
_a_, inner end of auditory meatus; _b_, partition dividing tympanic
cavity; _c_, styliform process.]
The medial surface (Fig. 24) presents in the middle near its ventral
margin a short triangular spine which lies in the natural state against
the ventral surface of the basilar portion of the occipital.
Caudad of this spine the surface is marked by two or three vertical
parallel grooves (Fig. 23, _m_). They indicate the portion of the bone
which bounds the jugular foramen, and are possibly impressions of the
ninth, tenth, and eleventh nerves.
The dorsal two-thirds of the medial surface is lacking in the
disarticulated bulla (Fig. 24), so that the cavity of the bone is
exposed. This opening is in the natural state closed by the petrous
bone. The caudal end is rough where it is overlaid by the jugular
process.
On the inner surface of the tympanic bulla is seen the thickened margin
of the inner end of the auditory meatus (Fig. 24, _a_). To it is
attached the membrana tympana. In the median dorsal line this margin is
notched for the reception of the incus and head of the malleus. From the
lateral wall of the cavity at the line of junction of the ectotympanic
and entotympanic a thin bony partition (_b_) rises. It runs almost
directly mediad; is concave dorsally and divides the tympanic cavity
into two chambers.
=The Petrous Portion= (Fig. 23, _b_, and Fig. 25).--This consists of two
parts, a very dense part (the petrous portion proper, Fig. 25), which
has the form of a triangular pyramid and encloses the auditory
labyrinth, and a less dense part, the =mastoid= portion (Figs. 22 and
23, _e_), which is flattened and triangular and is attached by its base
to the base of the pyramid.
The petrous portion may be described as having a base and three sides,
lateral, dorsal, and medial. It completes the medial wall of the
tympanic bulla, so that it is not possible to see it from the exterior
of a skull except through the auditory meatus (Fig. 22, _i_). When the
bones of the skull are articulated its dorsal surface is covered by the
tentorium and alisphenoid. Its lateral face looks into the tympanic
cavity, while the medial face looks into the cranial cavity.
[Illustration: FIG. 25.--PETROUS BONE OF RIGHT SIDE, LATERAL SURFACE,
ENLARGED.
_a_, fenestra cochleæ; _b_, promontory; _c_, fenestra vestibuli; _d_,
fossa for the tensor tympani muscle; _e_, fossa for incus and malleus;
_f_, fossa continuous with stylomastoid foramen; _g_, foramen leading
to facial canal.]
Its lateral face (Fig. 25) (medial wall of the tympanic cavity) presents
just ventrad of the middle of its base a large circular foramen, the
=fenestra cochleæ= (_a_) (or fenestra rotunda), which looks
caudolaterad; it leads into the cochlea. The fenestra cochleæ lies at
the summit of a nipple-like elevation, the =promontory= (_b_), which is
continued toward the apex of the bone as a gradually diminishing
semicylindrical ridge, due to the presence within it of the bony
cochlea. Dorsad of the fenestra cochleæ is the much smaller =fenestra
vestibuli= (_c_) (or ovalis) which leads into the vestibule. It is
occupied in the natural condition by the base of the stapes.
Dorsocraniad of the fenestra vestibuli is a large fossa (_d_) which
contains the tensor tympani muscle. Dorsocaudad of this, partly bounded
by the squamous portion of the temporal, is another large fossa (_e_),
the cranial end of which is occupied by the incus, while its caudal end
is occupied by the head of the malleus.
Nearly caudad of this fossa and separated from it by an oblique bony
septum is a third fossa (_f_) which is narrow and curved. It is
continuous with a notch in the mastoid portion of the bone. When the
tympanic is articulated the notch is converted into a foramen
(=stylomastoid foramen=, Fig. 22, _j_) for the exit of the seventh
nerve. The fossa gives passage to the seventh nerve and also lodges the
stapedius muscle. A groove may be traced from the stylomastoid foramen
to the caudal border of the fossa for the tensor tympani muscle, where
it passes into a canal (_g_). The groove and canal are parts of the
=canalis facialis= or =facial canal= (aqueductus Fallopii) for the
passage of the seventh nerve through the petrous bone.
The medial surface (Fig. 23, _b_) of the petrous portion shows near its
middle a fossa, the =internal auditory meatus= (_n_). This is divided by
a partition of bone into a dorsal and ventral part. The dorsal portion
is the beginning of the facial canal (aqueductus Fallopii) by which the
seventh nerve passes through the petrous bone to emerge at the
stylomastoid foramen. The ventral portion shows at its bottom several
small foramina for the auditory nerve.
Dorsocaudad of the internal auditory meatus is a deep fossa (_o_) for a
small lobe, the so-called appendicular lobe, of the cerebellum. This may
be called the appendicular fossa.
The dorsal surface is triangular and presents near its apex a
foramen--the =hiatus facialis= (_p_), the opening of a canal which joins
the canalis facialis and transmits the superficial petrosal branch of
the nerve of the pterygoid canal (Vidian nerve). That part of the dorsal
surface which lies caudad of the hiatus facialis is known as the =tegmen
tympani=.
The base of the petrous is attached to the mastoid portion (Fig. 23,
_e_).
(For an account of the structures within the petrous bone and the
tympanic cavity, see the description of the internal and middle ear.)
The =mastoid portion= (Figs. 22 and 23, _c_) is attached by its base to
the pyramidal petrous portion, with which it forms an angle of about 120
degrees. It appears in the lateral wall of the skull between the
parietal bone and the occipital (Fig. 40, _d_). The lambdoidal ridge is
continued on its outer surface to the caudal border of the external
auditory meatus. Caudad of the stylomastoid foramen it forms a slight
nipple-like eminence, the =mastoid process= (Fig. 22, _l_). Its inner
face looks into the cranial cavity.
=Parietal Bone.= =Os parietale= (Figs. 39, 40, and 43, 3).--The parietal
bones form the larger part of the lateral and dorsal boundary of the
cranial cavity. Each is a thin rectangular bone, compact and curved and
with a deeply notched shelf of bone, the =tentorium= (Fig. 42, _e_, and
Fig. 43, _f_), projecting inward from near the caudal margin.
The outer surface is smooth and convex. The highest part of the
convexity, a little caudad of the middle of the bone, is known as the
=parietal tubercle= or =eminence= (Fig. 39, _d_); it marks the point of
beginning ossification. An obscure curved ridge (Fig. 39, _e_), running
from the caudodorsal angle or a point craniad of it craniolaterad,
indicates the boundary of the origin of the temporal muscle. Near the
ventral border the surface is roughened and is covered in the natural
state by a part of the squamous portion of the temporal bone.
The inner surface (Fig. 43, 3 and 3′) is smooth and marked by ridges and
grooves for the convolutions of the cerebrum. Near the medial border is
a ridge which, when the bone is articulated with that of the opposite
side, forms a shallow groove for the =superior sagittal sinus=.
Beginning near the middle of the ventral margin and passing dorsad is a
groove for the middle meningeal artery. The =tentorium= (Fig. 43, _f_)
arises from the inner surface near its caudal margin and projects mediad
as a thin curved or notched shelf of bone which separates the cerebellar
fossa (Fig. 43, _I_) of the cranium from the cerebral fossa (Fig. 43,
_II_). When the parietals are articulated there is left between the
tentoria a large foramen by means of which the two fossæ communicate.
The foramen is bounded laterally and dorsally by the free margins of
the tentoria, while the ventral end of each tentorium articulates with
the alisphenoid, and its dorsal end with the opposite tentorium.
The medial border is straight and is united by suture to the opposite
bone.
The cranial border is bevelled at the expense of the inner surface and
articulates with the frontal. Just ventrad of the middle of the border
projects a sharp spine which fits into a corresponding notch in the
caudal border of the frontal.
The ventral border is concave, sharp, and bevelled at the expense of the
outer surface, for articulation with the squamous portion of the
temporal, except near the cranial end, where it articulates with the
wing of the sphenoid.
The caudal border is thick and porous medially, but thin laterally, and
bevelled at the expense of the inner surface for articulation with the
interparietal and mastoid portion of the temporal.
=Frontal Bone.= =Os frontale= (Figs. 39, 40, and 41, 5; Fig. 43, 8; Fig.
26).--The frontal bones meet one another in the median dorsal line so as
to form the roof of the skull between the parietal and nasal bones. A
part extends also ventrad, forming a large part of the medial wall of
the orbit and a part of the temporal fossa.
The bone may be divided into two portions, a plate forming the cranial
portion of the roof of the skull and a part of the roof of the nasal
cavity, the =frontal plate= (Fig. 40, 5), and a part descending into the
orbit, the =orbital plate= (Fig. 40, 5′).
The frontal plate (Fig. 40, 5) is a right-angled triangle with the
hypothenuse lateral. Its dorsal surface is convex and smooth. The
cranial two-thirds of its lateral border is separated from the orbital
fossa by a ridge, the =supraorbital arch= or margin (Fig. 39, _i_; Fig.
40, _o_); the caudal third passes gradually into the temporal fossa. At
its cranial angle is a triangular projection, the =frontal spine= or
nasal spine (Fig. 26, _a_), which fits into a space between the nasal
and maxillary bones.
[Illustration: FIG. 26.--FRONTAL BONE, MEDIAL SURFACE.
_a_, frontal spine; _b_, transverse ridge; _c_, surface applied to the
ethmoid; _d_, vertical plate of medial border.]
The ventral surface is concave and smooth over its caudal one-half and
helps to form the cranial part of the brain-case. It presents slight
ridges and depressions for convolutions of the cerebrum. At its narrowed
middle region the ventral surface is marked by a thick transverse ridge
(Fig. 26, _b_). Caudally the ridge descends by a gentle slope to the
level of the ventral surface of the bone. The cranial end of the ridge
is pierced by an oval foramen through which the frontal sinus (Fig. 43,
_m_, _m′_), which lies within the ridge, communicates with the spaces in
the ethmoid bone (nasal cavity). Craniad of the ridge the surface (Fig.
26, _c_) is rough and, together with the raised medial border of the
bone and the orbital plate, encloses a rectangular space which in the
natural state receives a portion of the labyrinth of the ethmoid. The
ventral surface is marked at its medial edge by a thin longitudinal
ridge which, when the bones are articulated, is continuous with one of
the vertical lamellæ of the ethmoid.
The medial border forms a vertical plate (_d_), broadest craniad and
roughened for articulation with its fellow of the opposite side except
at its cranial end, where it articulates with the border of the nasal
bone.
The caudal border is roughened, bevelled at the expense of the outer
surface, and articulated with the parietal bone except at its ventral
end, where it articulates with the alisphenoid.
The lateral border is smooth, and it is here that the orbital plate is
joined to the frontal plate at right angles. Along its cranial
two-thirds this union is marked by a sharp ridge, the supraorbital
margin (Fig. 40, _o_) or arch. This ridge extends caudolaterad as a
triangular projection, the =zygomatic= (or postorbital) =process= (Fig.
40, _n_), which is flattened on its cranioventral face near its
extremity and forms part of the boundary of the orbital fossa. At its
cranial end the lateral border articulates with the nasal and maxillary
bones.
The orbital plate (Fig. 40, 5′) arises from the ventral surface of the
lateral border of the frontal plate. It is directed ventrad, is smooth
and concave on its outer surface, and forms the dorsal portion of the
medial wall of the orbital fossa. Near its ventral border it bears the
small =ethmoidal foramen=, for the artery of the same name.
On the caudal one-half of its inner surface (Fig. 26) it assists the
caudal part of the dorsal plate in forming the brain-case. The cranial
one-half of its inner surface is marked off from the remainder of the
surface by a sharp irregular ridge which is for articulation with the
cribriform plate of the ethmoid. Craniad of this the surface is marked
by ridges and looks into the nasal cavity.
The cranial margin is produced dorsally in the form of a blunt
triangular spine. Mediad of this spine the bone articulates with the
lachrymal bone.
The ventral border articulates by its cranial one-third with the orbital
plate of the palatine, and by its caudal two-thirds with the body and
wing of the presphenoid.
=Maxillary Bone.= =Maxilla= (Figs. 27 and 28).--The maxillary bone forms
the cranial and lateral portions of the roof of the mouth. The bones of
opposite sides meet craniad, but diverge caudad to enclose the palatal
plates of the palatine bones. Each consists of a thick prismatic ventral
portion or body (_a_) and a thin flat plate, the =frontal process=
(_b_), extending dorsad from the cranial part of the bone.
[Illustration: FIG. 27.--MAXILLARY BONE, LATERAL SURFACE.