Part 10

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.

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.

FIG. 28.--MAXILLARY BONE, MEDIAL SURFACE.