Chapter IV: The Skull (1)

The bones of the head consist of the =skull proper= together with a number of separate bones forming part of the =visceral skeleton=; these are the lower jaw, the hyoid, and the ear-bones.

The skull proper is considered as divided into =cranial= and =facial= portions. The former includes all the bones which take part in bounding the cranial cavity or cavity of the brain; the latter includes the bones which support the face.

The cranial portion of the skull includes all that part enclosing the large cavity which contains the brain. For convenience this portion may be considered as made up of three segments, each of which forms a ring surrounding a part of the cranial cavity. The first or caudal segment or ring consists of the occipital bone (with the interparietal) surrounding the foramen magnum. The second segment consists of the sphenoid ventrad, the parietals laterad and dorsad. Between the first and second segments are intercalated laterally the temporal bones containing the auditory organ. The third segment or ring consists of the presphenoid ventrad, of the frontals laterad and dorsad. The cranial opening of this ring is closed by the lamina cribrosa of the ethmoid.

The cranial portion of the skull therefore contains eleven separate bones: one occipital, one interparietal, two temporals, one sphenoid, two parietals, one presphenoid, two frontals, and one ethmoid.

The facial portion of the skull is much smaller than the cranial, and lies craniad of the latter; it encloses the nasal cavity. It contains the following thirteen bones: two palatines, one vomer, two maxillaries, two lachrymals, two premaxillaries, two nasals, two malar or zygomatic bones. The two halves of the mandible or lower jaw are frequently included in the facial portion of the skull, making in all fifteen separate bones in this part of the skull.

In the following the bones of the skull are first described separately, then an account is given of the skull as a whole.

=Occipital Bone.= =Os occipitale= (Figs. 17 and 18).--The occipital bone forms the most caudal portion of the cranium, entering into the formation of its caudal wall and of its base. It connects the cranium with the vertebral column and surrounds a large opening, the =foramen magnum= (_d_), by means of which the cranial cavity communicates with the vertebral canal.

FIG. 18.--OCCIPITAL BONE, INNER SURFACE.

_a_, basilar portion; _b_, lateral portions; _c_, squamous portion;
_d_, foramen magnum; _e_, occipital condyles; _f_, jugular processes;
_g_, jugular notch; _h_, lambdoidal ridge; _i_, external occipital
crest; _j_, external occipital protuberance.]

In young kittens four portions may be distinguished in this bone. These are, a basal portion (the =basioccipital bone=), two lateral portions (the =exoccipital bones=), and a dorsal portion (the =supraoccipital bone=). These four bones remain separate through life in many lower vertebrates. In the adult cat they are completely united into a single bone, but it is convenient to describe this bone as made up of four parts: a basal portion (_a_) (=pars basilare=), corresponding to the basioccipital, two lateral portions (_b_) (=partes laterales=), corresponding to the exoccipitals, and a squamous portion (_c_) (=squama occipitalis=) corresponding to the supraoccipital.

The basilar portion (_a_) as seen from the dorsal or ventral surface is oblong and flattened. It is broadest at the junction of its middle and last thirds, and tapers toward both ends. It presents a cranial end and a caudal end, a dorsal, a ventral, and two lateral surfaces. Its caudal end, which helps to form the ventral boundary of the foramen magnum, is concave from side to side. The cranial end is transversely elongate, about five times as broad as high, pointed laterally and roughened for attachment to the caudal end of the body of the sphenoid, which has a corresponding form. The joint is a synchondrosis.

The dorsal surface is concave from side to side, forming a longitudinal groove in which rest the pons and medulla. The concavity is more pronounced caudad, where the bone is thinner at its middle.

The ventral surface is marked by three parallel longitudinal ridges. One of them is median and expands caudad into a smooth triangular elevated area which extends to the foramen magnum. The other two ridges run near the lateral edges of the bone. Between them and the median ridge the surface is smooth and is depressed caudad. Laterad of each lateral ridge is a rough triangular surface overlaid in the natural state by the medial edge of the tympanic bulla.

The lateral surfaces are smooth and sharp and abut against the petrous portion of the temporal bone. They pass into the lateral portions of the occipital caudad.

The =lateral portions= (_b_) of the occipital arise from the caudal margin of the basilar portion in the transverse plane; a short distance laterad of the line of junction they turn dorsad at an angle of nearly ninety degrees. They form the lateral boundaries of the foramen magnum and pass dorsally into the squamous portion.

The external surface of each presents an elongated elevated spirally curved surface, the =occipital condyle= (_c_), for articulation with the atlas. A small part of each condyle is formed from the basal portion. The two condyles are separated from one another ventrally by a narrow notch, and each extends (laterad) along the border of the foramen magnum to a point slightly dorsad of the transverse diameter of the latter.

Laterad of each condyle the bone is elevated into a blunt triangular projection, the =jugular process= (_f_), which covers the caudal end of the tympanic bulla. Between the jugular process and the condyle is a deep depression.

The internal surface (Fig. 18) is concave dorsoventrally, following the outline of the foramen magnum. It is convex from side to side. It is smooth except at its outer margin, which is rough for articulation with the mastoid portion of the temporal bone. The cranial face of the jugular process (_f_) shows a rough concavity for the reception of the bulla tympani. Mediad of the jugular process is a notch (=jugular notch=) (_g_) which when the bones are articulated forms part of the boundary of the jugular foramen. Mediad of this notch is a foramen which forms one end of the =hypoglossal canal=. It passes dorsocaudad into the cranial cavity and transmits the hypoglossal nerve. Dorsad of the hypoglossal canal is the cranial opening of the =condyloid canal=, which passes caudad and opens just craniad of the dorsal end of the condyle. It transmits a vein. The outer border of this portion is rough for articulation with the petrous and mastoid portions of the temporal.

The =squamous portion= (_c_) has the form of a sector of a circle whose arc is a little more than ninety degrees. The central angle of the sector is truncated and bounds the foramen magnum dorsally. The arc of the sector forms the dorsal margin of the bone, while along the radii it passes into the lateral portions. Its dorsal portion is thick and porous; its ventral portion near the foramen magnum is thin and compact.

The external surface (Fig. 17) is marked by a prominent ridge, the =lambdoidal ridge= (_h_), parallel with the dorsal border and near to it. The narrow portion of the outer surface that lies dorsad of the ridge forms an angle of about ninety degrees with the remainder of the surface.

A median crest (_i_) extends ventrad from the middle of the lambdoidal ridge toward the foramen magnum; this is the =external occipital crest=. At its junction with the lambdoidal ridge it is elevated into a tubercle, the =external occipital protuberance= (_j_).

The inner surface (Fig. 18) presents depressions for the convolutions of the cerebellum.

The dorsal border is thick and rough for articulation with the parietals and interparietal. The ventral border abuts on the foramen magnum and is thin and smooth.

=Interparietal Bone.= =Os interparietale= (Fig. 19).--This is a small triangular bone lying between the parietals, with its apex directed craniad, and its base in contact with the squamous portion of the occipital.

Its dorsal surface is arrow-shaped and has its posterior border notched. It is marked by a median crest (part of the =sagittal crest=) which is continued craniad from the middle of the lambdoidal crest.

The ventral surface is irregularly triangular, smooth, and concave. The three borders are rough for articulation with the parietals and occipital.

FIG. 20.--SPHENOID BONE, DORSAL OR INTERNAL SURFACE.

Fig. 19, showing the sagittal crest running craniocaudad across its
middle.

Fig. 20.--_a_, body; _b_, wings; _c_, pterygoid process; _d_,
tuberculum sellæ; _e_, dorsum sellæ; _f_, sella turcica; _g_, notch
which aids in forming the foramen lacerum; _h_, longitudinal groove of
alisphenoid; _i_, notch which aids in forming the orbital fissure;
_j_, foramen rotundum; _k_, foramen ovale.]

=Sphenoid Bone.= =Os sphenoidale= (Fig. 20).--The sphenoid bone of man is represented in the cat by two entirely distinct bones,--one cranial, the other caudal. The cranial portion may be designated as the =presphenoid= (Fig. 21); the caudal part will be described as the =sphenoid= (proper) (Fig. 20).

The sphenoid bone in the kitten is in three parts: a central portion, the =basisphenoid=, and two lateral portions, the =alisphenoids=. In many lower vertebrates these three bones are permanently distinct, but in the adult cat they are united to form the sphenoid bone. To these there is added a fourth element, separate in many vertebrates as the =pterygoid= bone. The sphenoid may thus be described as composed of a central portion, the body (_a_) (basisphenoid), and of two thin expanded wings (_b_) (alisphenoids, alæ magnæ of the human sphenoid); each of which has arising from it a thin curved process, the =pterygoid process= (_c_), directed craniad and largely made up of the pterygoid bone.

The body of the sphenoid (_a_) lies in the middle line of the base of the skull. It is wedge-shaped, with the converging sides of the wedge directed laterad and its apex pointed craniad.

It has six surfaces, of which the dorsal and a part of the laterals look into the cranial cavity. The cranial end articulates with the body of the presphenoid, and the caudal with the body of the occipital.

The dorsal surface is triangular, with one apex of the triangle truncated, elevated, and directed craniad. This elevation is the =tuberculum sellæ= (_d_). Just caudad of the middle the surface presents a rectangular elevation with rounded angles, the =dorsum sellæ= (_e_). The cranial end of the dorsum sellæ presents at each dorsolateral angle a very small smooth tubercle which represents one of the posterior clinoid processes of man. Between this elevation and the elevated cranial end of this surface there is a deep excavation, the =sella turcica= (_f_), in which in the natural state is lodged the hypophysis. Near the cranial end of the sella is a small foramen, probably nutrient. At the caudal end of the body a slight notch (_g_) separates it from the wing: this notch forms a part of the =foramen lacerum=. Against this notch fits the apex of the petrous bone, and from it a groove (=carotid groove=) is continued mediocraniad to the sella turcica.

The ventral surface (Fig. 41, 3) is triangular, smooth, and nearly flat; it is marked by a median ridge which is the continuation craniad of the ridge on the ventral face of the basilar portion of the occipital.

Its caudal angles are separated from the rest of the bone by sharp triangular elevations, laterad of which are rough triangular areas, overlaid when the bones are articulated by a triangular spine from the tympanic bulla.

Its lateral surfaces are mostly covered by the wings. They appear at the sides of the elevated cranial end of the dorsal surface as triangular areas.

The caudal end is concave, rough, and has the form of the cranial end of the basilar part of the occipital.

The cranial end is nearly square and rough for articulation with the body of the presphenoid.

=The Wing= (=alisphenoid=; ala magna of the human sphenoid) (Fig. 20, _b_).--This is a thin quadrilateral plate of bone attached by its medial border to nearly the whole of the lateral surface of the body. Its middle portion lies nearly in the same plane as the body, but its ends are curved dorsad so that its internal surface is concave and its external surface is convex. The curvature is most pronounced near the long lateral border, so that this border forms nearly a semicircle.

The internal surface supports the occipital lobe of the cerebrum. It is marked by a rounded groove (=h=) which is parallel with the lateral surface of the body. The dorsal margin of the groove projects mediad in the form of a sharp ridge which is broadest caudad, where it often reaches nearly to the posterior clinoid process. The groove passes craniad into three foramina. The first (cranial) of these, the =orbital fissure= (_i_), is large and lies between the wing, the body, and the pterygoid process. It is incomplete, but is completed by the presphenoid. The second foramen is small and rounded; it is the =foramen rotundum= (_j_). The third, =foramen ovale= (_k_), is larger and oval and penetrates the wing through about the middle of its longitudinal axis. Another minute foramen penetrates the sphenoid between the wing and the body of the bone, just laterad of the tuberculum sellæ. This foramen is continuous craniad with a groove on the dorsal surface of the pterygoid process; the groove and foramen constitute the =pterygoid canal=. It transmits a nerve.

The external surface shows the orbital fissure, the foramen rotundum and the foramen ovale, bounded ventrally by a sharp ridge, which is continued onto the pterygoid process. Between this ridge and the body the surface is longitudinally grooved for the tuba auditiva or Eustachian tube.

The semicircular margin of the bone articulates with the squamous portion of the temporal. At the junction of its caudal and middle third there is sometimes a toothlike projection which underlies the root of the zygoma.

The whole of the cranial margin, except the lateral end, articulates with the wing of the presphenoid. At this end the angle formed by the junction of lateral and cranial borders is produced into a flat process, which passes dorsocaudad between the squamous portion of the temporal and the frontal, and articulates by the roughened internal surface of its free end with a similar process from the parietal.

The caudal margin laterad of the groove is bevelled and roughened at the expense of the dorsal surface and is overlaid by the ventral end of the tentorium. Mediad of the groove it projects caudad as a slender point, the =lingula= of the sphenoid. This is received into a narrow cleft between the apex of the petrous bone and the bulla tympani.

The pterygoid process (_c_) is a nearly square, thin plate of bone. The medial surface is smooth and concave, the lateral face is convex and marked by two parallel ridges. The medial one of these is continued craniad from the bony septum which separates the orbital fissure from the foramen rotundum, and the lateral one from the septum which separates the foramen rotundum from the foramen ovale. A sharp triangular spine projects laterad from near the caudal end of the lateral ridge.

The two ridges and that part of the lateral surface of the bone included between them form a part of the sphenoid bone known as the pterygoid process of the sphenoid bone, in those cases where the pterygoid is a separate bone.

The remainder of the process is equivalent to the pterygoid bone of other vertebrates.

Between the caudal margin of this bone and the lateral of the two ridges, i.e., between the pterygoid bone and the pterygoid process of the sphenoid, is a long deep fossa, the =internal pterygoid fossa= (Fig. 40, _s_). The laterocaudal margin of the pterygoid process projects caudad, as a curved triangular spine, the =hamulus= or =hamular process= (Fig. 40, _t_; Fig. 43, _i_).

The =Presphenoid Bone=. =Os presphenoidale= (Fig. 21).--In a young cat this bone is in three pieces, a basal portion (=presphenoid=) and two wings (=orbitosphenoid bones=). These bones remain distinct throughout life in many lower vertebrates, but in the adult cat they fuse to form a single bone. We may nevertheless conveniently describe this bone as made up of a body (_a_) (the basisphenoid), and two wings (_b_), the orbitosphenoids (the alæ parvæ of the human sphenoid).

_a_, body; _b_, wings; _c_, optic foramina.]

The body (_a_) lies in the base of the skull in the median line, craniad of the basisphenoid. It has the form of a rectangular prism about twice as long as broad. It is hollow, and the cavity is divided by a median longitudinal partition into two cavities (=sphenoidal sinuses=, Fig. 43, _l_). The sphenoidal sinuses are continued craniad into the cavities of the ethmoid. The body has six surfaces:

The dorsal or internal surface (Fig. 42, _n_) looks into the cranial cavity and is continuous with the dorsal surface of the wings. The caudal end of the body is depressed, and when united to the basisphenoid aids in forming the cranial wall of the sella turcica. At each caudolateral angle is a short spine, the =anterior clinoid process=. At about one-third the length of the bone from the caudal end is a transverse groove (=chiasmatic groove=, Fig. 42, _m_) for the optic chiasma. Its ends lead into two round foramina (the =optic foramina=, Fig. 42, _l_; Fig. 21, _c_) which pass craniolaterad between the body and the wings of the presphenoid and transmit the optic nerve and the ophthalmic artery.

The ventral surface (Fig. 21) is hour-glass-shaped and marked by a smooth median ridge, continuous with the ridge on the basisphenoid and overlaid at its cranial end by the vomer. The caudal end presents a rough triangular area on each side, for articulation with the pterygoid process of the sphenoid bone, while the cranial end has similar areas overlaid by the nasal portion of the palatine bones.

The lateral surface looks towards the orbitotemporal fossa. It is notched near the caudal end by the ventral border of the optic foramen (_c_). Caudad of this foramen the surface presents an oblique groove which forms in the natural condition the medial boundary of the orbital fissure.

Craniad of the optic foramen the surface is smooth and marked near its ventral border by a longitudinal ridge which forms part of the dorsal boundary of a fossa, the =external pterygoid fossa= (Fig. 40, _p_).

The caudal end presents ventrally a quadrangular rough surface for articulation with the body of the sphenoid. The cranial end presents the two sphenoidal sinuses separated by a median partition.

The median partition articulates by its free border with the lamina perpendicularis of the ethmoid. At its ventral end is the abruptly truncate end of the median ridge of the ventral surface, which is continuous with the ventral cartilaginous portion of the lamina perpendicularis. The lateral walls of the sphenoidal sinuses are continued craniad of the dorsal and ventral walls and of the median partition, and articulate ventrally with the nasal portion of the palatine bones, and dorsally with the orbital plate of the frontal. Between them is received the caudal ends of the labyrinths of the ethmoid in the middle, while between their dorsal edges is received the caudal end of the cribriform plate, and between their ventral edges the expanded end of the vomer.

The =wings= (_b_) arise each from nearly the whole of the dorsolateral angle of the body. They form prominent nearly horizontal triangular projections over the optic foramina.

The dorsal and ventral surfaces are smooth and continuous respectively with the dorsal and lateral surfaces of the body. The dorsal surface looks into the cranial cavity, while the ventral looks into the orbitotemporal fossa.

Craniad of the apex of the wing its border articulates with the ventral edge of the orbital portion of the frontal bone. Caudad of the apex the border articulates with the cranial border of the wing of the sphenoid.

=Temporal Bone.= =Os temporale= (Figs. 22 and 23).--This forms a part of the lateral wall of the cranium, filling the gap between the occipital and the sphenoidal segments. It is made up of three portions which are distinct in kittens but somewhat firmly united in adult cats. In lower vertebrates these portions are distinct bones called the =Squamous=, the =Petrous=, and the =Tympanic= bones. In the cat they may be described as the squamous (_a_), petrous (_b_), and tympanic (_c_) portions of the temporal bone.

FIG. 23.--TEMPORAL BONE, INTERNAL SURFACE.

_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.

_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.

_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.

_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.

_a_, body; _b_, frontal process; _c_, infraorbital foramen; _d_,
elevation for root of canine tooth; _e_, canine tooth; _f_, first
premolar; _g_, second premolar; _h_, third premolar; _i_, molar tooth;
_j_, zygomatic process; _k_, beginning of lachrymal canal; _l_, ridge
to which the ventral nasal concha is attached; _m_, nasal crest of
palatine process.]

The body (_a_) has the form of a triangular prism whose broader dorsal face looks into the nasal cavity and orbit, while the ventral face looks into the mouth, and the lateral face toward the cheek. From the junction of the dorsal and lateral surfaces at the cranial end the large flat curved frontal process (_b_) passes dorsad, while the teeth are implanted along the border, =alveolar border= or process, formed by the junction of the ventral and lateral surfaces.

The lateral surface is continuous with the lateral surface of the frontal process and shows at the base of the frontal process on its caudal border the large =infraorbital= foramen (Fig. 27, _c_), for the vessels and nerves of the same name. Near the medial end of the surface is a cylindrical elevation (_d_) for the root of the canine tooth (_e_).

The ventral surface is smooth and looks into the roof of the mouth.

On the dorsal surface caudal and cranial halves may be distinguished. The caudal one-half enters into the floor of the orbit. The lateral edge of this portion is divided into two laminæ, between which is received the end of the malar bone. Caudad this edge is prolonged into the short dorsally directed =zygomatic process= (_j_). The cranial half of the dorsal surface looks into the nasal cavity and is separated from the caudal half by a sharp vertical lamina of bone which runs caudomediad from the base of the nasal process. To the dorsal edge of this lamina are articulated the lachrymal bone and a part of the palatine. At the point where the lamina joins the base of the nasal process a foramen is seen leading into a canal, the =nasolachrymal canal= (_k_). Craniad of the lamina the surface is concave. Where it becomes continuous with the inner edge of the frontal process there is attached to it a thin bone, the =ventral nasal concha= (or maxilloturbinal), which is rolled into an irregular spiral. The nasolachrymal canal opens ventrad of its cranial end.

The cranial third of this part of the bone projects further mediad than does the rest of the medial border, forming thus the broad =palatine process=. This is rough on its medial edge for articulation with the premaxillary and the palatine process of the opposite bone. This medial edge rises also dorsally into a low ridge, the =nasal crest= (_m_), which is roughened for articulation with the vomer. The caudal two-thirds of the medial edge articulates with the palatine bone.

The cranial end of the bone articulates with the premaxilla.

The caudal end is smooth.

The frontal process (_b_) presents on its inner surface, which looks into the nasal cavity, certain transverse ridges which are in relation with the ethmoid bone. Its outer surface is smooth. By its cranial border it articulates with the nasal bone dorsally and with the premaxillary bone ventrally.

_a_, the three incisor teeth; _b_, palatal portion of the bone; _c_,
nasal process.]

Its dorsal end articulates medially with the nasal spine of the frontal bone, and caudally with the orbital plate of the same bone.

=Premaxillary Bone.= (=Os incisivum BNA.=) =Premaxilla= (Fig. 29).--The premaxillary bones bear the incisor teeth and form the cranial portion of the roof of the mouth.

Each consists of an irregular, horizontal =palatal portion= (_b_) and of a perpendicular =nasal process= (_c_) which forms part of the lateral boundary of the nares and enters into the formation of the lateral wall of the nasal cavity.

The palatal portion has in its caudal border a deep notch for the foramen incisivum or anterior palatine canal, which lies between it and the maxillary and transmits blood-vessels and nerves. It articulates with the maxillary bone by this border.

The medial border is raised into a thin crest of bone which, besides forming the medial wall of the foramen incisivum or anterior palatine canal, articulates by its medial border with the bone of the opposite side, forming a sort of median trough (=sulcus palatinus=) which projects dorsad into the nasal cavity and receives the ventral border of the nasal septum. The caudal end of this border articulates laterad with the maxilla, dorsad with the vomer.

Its craniolateral border bears the incisor teeth (_a_).

The nasal process (_c_) presents three surfaces, all elongated and triangular; one, the medial surface, is smooth and concave and looks into the nasal cavity. Its dorsal border is rough for articulation with the nasal bone dorsad, and smooth ventrad where it aids in forming the nares.

The lateral surface is smooth.

The caudal surface is rough for articulation with the maxillary bone.

=Nasal Bone.= =Os nasale= (Fig. 30).--The nasal bones fill the space between the nasal process of the premaxillary, the frontal process of the maxillary, and the nasal spine of the frontal bone (Fig. 39, 7). They thus form part of the dorsal wall of the nasal cavity near the middle line.

Each may be described as consisting of two elongated triangular lamellæ, one vertical, the other horizontal. The vertical lamella is curved slightly ventrad and has its apex directed craniad. It is applied by its medial surface against the vertical lamella of the opposite bone, the two thus forming a median vertical partition, the =nasal crest= (Fig. 43, 12), which extends ventrad into the nasal cavity and, by joining the dorsal edge of the lamina perpendicularis, helps to form the internasal septum.

The horizontal lamella is attached to the dorsal margin of the vertical lamella in such a way that its apex lies opposite the base of the vertical lamella. It helps to roof in the nasal cavity, and by its base forms a part of the dorsal boundary of the narial opening. By its lateral margin it articulates with the nasal spine of the frontal at its caudal end, with the frontal process of the maxillary at its middle, and with the nasal process of the premaxilla at its cranial end. The lateral angle of its base projects in a curved line which forms the dorsal part of the lateral boundary of the narial opening.

From the lateral border of the horizontal lamella a bony plate curves ventrad and mediad, enclosing a narrow fossa which receives a part of the ethmoid. This is the =concha nasalis superior= (nasoturbinal bone).

=Ethmoid Bone.= =Os ethmoidale= (Figs. 31 and 32).--The ethmoid bone closes in the cranial cavity at its cranial end and extends forward into the nasal cavity, which it largely fills.

It consists of a median vertical portion, the =lamina perpendicularis= (Fig. 43, _n_; Fig. 42, _p_), forming a part of the nasal septum, of two lateral portions made of thin sheets of bone variously folded and united--the =labyrinths= (or ethmoturbinals), which fill the greater part of the nasal cavity; and of a transverse perforated plate, the =cribriform plate= (lamina cribrosa), attached to the caudal end of the lamina perpendicularis and the labyrinths.

The lamina perpendicularis (Fig. 43, _n_; Fig. 42, _p_) is a flat four-sided bone. By its caudal margin it is continuous with the cribriform plate; by its ventral margin it is enclosed by the halves of the vomer; by its dorsal margin it unites with the crest formed by the vertical portion of the nasal bone craniad and with the vertical lamina of the medial margin of the frontal caudad, while its cranial margin is continued into the septal cartilage of the nose. Its lateral faces are smooth and free.

The lamina cribrosa or cribriform plate (Fig. 42, _o_) is elongated heart-shaped, with the apex of the heart ventrad. Its caudal face is concave and looks into the cranial cavity. It presents three irregular longitudinal rows of holes, one median and two lateral, for the passage of the olfactory fibres from the cranial cavity into the nasal cavity. Its cranial face is continuous along the medial line with the lamina perpendicularis, and at the sides with the labyrinths.

The notch in the heart is directed dorsad and receives the vertical lamina of the medial border of the frontal bone. The apex of the heart articulates with the cranial end of the dorsal surface of the presphenoid. Its lateral margins are articulated with the ethmoidal ridges on the medial surface of the frontal bone.

FIG. 32.--ETHMOID AND VOMER, VENTRAL VIEW.

_a_, vomer; _b_, vertical cells of the labyrinth of the ethmoid; _c_,
horizontal cell of the same; _d_, part of the ethmoid that forms the
lamina papyracea; _e_, edge of cribriform plate.]

The labyrinths (Figs. 31 and 32) are attached to the cranial face of the lamina cribrosa, one on each side of the lamina perpendicularis. Each is made of thin bony plates irregularly folded so as to enclose spaces, the =ethmoid cells=. In each may be distinguished a cranial portion (_b_), in which the cells are nearly vertical, and a caudal portion (_c_), in which the cells are nearly horizontal.

The medial surfaces are separated by a space from the lamina perpendicularis. This space is broadest along the junction of the horizontal and vertical portions of the labyrinth. There are thus formed two passageways which correspond to the superior meati of human anatomy.

The lateral surfaces come into contact with the frontal process of the maxillary and the orbital plate of the frontal bone. On the lateral surface of each labyrinth there is a thin irregular lamina of bone lying in a dorsoventral longitudinal plane and closing in some of the ethmoid cells laterally (_d_). A small part of this lamina, situated near the caudoventral angle of the bone, appears in the orbital fossa on the external surface of the skull between the presphenoid, palatine, and frontal bones or between the lachrymal, palatine, and frontal bones. Sometimes in the entire skull two such pieces may be seen, one in each of these positions. This corresponds to the =lamina papyracea= of human anatomy.

The dorsocaudal angle of each bone is received into the space between the orbital plate of the frontal and the vertical lamina of the medial border of the frontal. Its ventrocaudal angle is received between the cranial extensions of the lateral walls of the presphenoid, while its ventral surface is overlaid caudally by the expanded portion of the vomer, to which it is attached at its caudolateral angles.

=Vomer= (Figs. 31 and 32, _a_).--The vomer consists of two thin laminæ of bone which ensheath the ventral margin of the lamina perpendicularis (or the cartilaginous plate which continues ventrad from this margin) and unite ventrad of it; the two thus form a trough open dorsad.

Each becomes horizontal near its caudal end and at the same time expands. The expanded portion lies ventrad of the labyrinth of the ethmoid, closing in some of its cells: its lateral angles are united with the labyrinths.

At its caudal end the bone articulates with the body of the presphenoid, and each half of it is produced caudad near the middle line into a triangular spine which lies ventrad of the body of the presphenoid. The horizontal portion of the bone helps to separate the olfactory and respiratory passages of the nasal chamber, while its vertical portion contributes to the formation of the nasal septum.

The ventral margin formed by the junction of the two halves of the bone is smooth and free caudad, but at its cranial end is broad and rough for articulation with the palatal processes of the maxillæ.

=Palatine Bone.= =Os palatinum= (Fig. 33).--The palate bone or palatine bone consists of two portions, a =horizontal= or =palatal= portion (_a_) and a =perpendicular= or =nasal= portion (_b_), uniting at an angle of about forty-five degrees.

_a_, horizontal portion; _b_, perpendicular portion; _c_, maxillary
spine; _d_, posterior nasal spine; _e_, sphenopalatine foramen; _f_,
caudal opening of posterior palatine canal.]

The horizontal portions (_a_) of the two bones are received between the maxillary bones and form the caudal and medial part of the roof of the mouth. Each is irregularly quadrilateral in form, with the caudolateral angle produced caudad into a long process which is continuous with the perpendicular portion of the bone. The lateral margin of the horizontal portion articulates over its cranial half with the maxillary bone. At about its middle a short thick =maxillary spine= (_c_) projects caudolaterad. The remainder of the lateral margin is directly continuous with the perpendicular plate of the bone. The medial margin is rough for articulation with the corresponding margin of the opposite palatine; the caudal angle of this margin projects caudad as the short =posterior nasal spine= (_d_). The caudal margin forms a free edge which bounds the choanæ; it passes laterally into the perpendicular portion.

The ventral surface (Fig. 41, 8) looks into the mouth. Near the middle of its craniolateral margin are two or more small foramina (Fig. 41, _q_) which form the cranial termination of the posterior palatine canal. The dorsal surface is smooth and looks into the nasal cavity.

The perpendicular or nasal portion (Fig. 33, _b_) of the palatine is thin and irregularly quadrilateral in form. It is attached by its cranial two-thirds to the dorsal surface of the horizontal portion. The outer surface is concave and looks into the orbital fossa. The inner surface is convex and looks into the nasal cavity.

The perpendicular portion is marked by two foramina just craniad of the middle. The larger dorsal oval foramen is the =sphenopalatine= foramen (_e_). The smaller ventral foramen is the caudal opening of the posterior palatine canal (_f_). From this opening the canal passes craniomediad, lying in the substance of the palatine bone; it opens on the ventral surface of the horizontal portion at the small openings previously described (Fig. 41, _q_).

By its cranial margin it articulates with the lachrymal bone. By its dorsal margin it articulates craniad with the orbital plate of the frontal: with the lamina papyracea at its middle, and with the body of the presphenoid caudad. The caudal half of the dorsal margin is partially divided into two lamellæ with a rough surface between them: this rough surface lies against the ventral surface of the presphenoid. The caudal margin articulates with the pterygoid portion of the sphenoid.

=Lachrymal Bone.= =Os lachrymale= (Fig. 34; Fig. 39, 10).--The lachrymal bone is a thin pentagonal scale of bone filling the interval between the horizontal plate of the palatine, the maxillary, and the orbital plate of the frontal. Its outer surface looks into the orbit, its inner surface into the nasal cavity.

FIG. 35.--MALAR BONE OF RIGHT SIDE, LATERAL SURFACE.

Fig. 34.--_a_, notch forming the beginning of the lachrymal canal.

Fig. 35.--_a_, ridge for origin of the masseter muscle; _b_, frontal
process; _c_, zygomatic process.]

Near the middle of its cranial border it is notched obliquely by a foramen (_a_), the beginning of the nasolachrymal canal.

=Malar Bone.= =Jugal Bone.= =Os zygomaticum= (Fig. 35).--The malar or zygomatic bone is a flat curved plate of bone which forms the lateral wall of the orbit and together with the zygomatic process of the temporal forms the zygomatic arch. Its outer surface is smooth and marked by a longitudinal ridge (_a_) for attachment of the masseter muscle.