Chapter IV: The Skull (2)

At its caudal end the bone is continued into two processes: one, the =frontal= process or =orbital= process (_b_), is a triangular spine of bone directed caudomediad; when the bones are articulated it lies opposite the zygomatic process of the frontal to which it is joined by a ligament (orbital ligament). The other, =zygomatic= process (_c_) of the malar bone, extends ventrocaudad and articulates with a similar process from the temporal to form the zygomatic arch above mentioned.

Its inner surface is smooth and looks into the orbit, except that of the zygomatic process, which looks into the temporal fossa.

Its cranial border is roughened at the expense of both surfaces and articulates with the maxillary bone. Its other borders are smooth except the dorsal border of the zygomatic process, which is roughened for attachment to the zygomatic process of the temporal.

=The Mandible.= =Mandibula= (Figs. 36 and 37).--The mandible (or inferior maxillary bone) is composed of two halves which come together at the cranial end and form the lower jaw. At its caudal end each half articulates with the temporal bone at the mandibular fossa, and at its cranial end it joins the opposite bone, the suture being known as the =symphysis= of the jaw (=symphysis menti=) (Fig. 37, _a_).

Each half consists of a horizontal portion, the =body= (_b_), bearing teeth on one of its borders (the =alveolar border=), and of a vertical portion, the =ramus= (_c_).

The body (_b_) has the form of a flattened cylinder and has two surfaces and two borders. The lateral surface (Fig. 36) is smooth and presents near its cranial end a foramen (or sometimes two), the =mental foramen= (_d_), forming the cranial termination of the mandibular canal. At its caudal end is a deep fossa continuing on to the ramus, the =coronoid fossa=, or =masseteric fossa= (_e_).

FIG. 37.--MANDIBLE, MEDIAL SURFACE.

_a_, symphysis; _b_, body; _c_, ramus; _d_, mental foramina; _e_,
coronoid fossa; _f_, mandibular foramen; _g_, angular process; _h_,
coronoid process; _i_, condyloid process; 1, 2, 3, the three incisor
teeth; 4, the canine tooth; 5, 6, the premolars; 7, the molar tooth.]

The medial surface (Fig. 37) is smooth and has near its caudal end a foramen, the =mandibular foramen= (_f_), which communicates with the mandibular canal leading lengthwise through the bone to the mental foramen. The cranial end is roughened for attachment to the bone of the opposite side.

The ventral border is smooth and rounded; it ends caudally in a blunt point, the =angular process= (_g_). The dorsal (alveolar) border is slightly curved and bears the sockets (=alveoli=) for the teeth. It is continuous with the cranial margin of the coronoid process.

The ramus is divided into two portions, the =coronoid= process (_h_) and the =condyloid= process (_i_). The coronoid process (_h_) extends dorsocaudad as a thin plate of bone with smooth surfaces and borders. Its outer surface is partly occupied by the =coronoid fossa= (_e_). The condyloid process (_i_) has the form of a semicylindrical transverse piece of bone attached to the caudal margin of the coronoid process. It articulates with the mandibular fossa of the temporal bone.

=Hyoid Bone.= =Os hyoideum= (Fig. 38 and Fig. 104).--The hyoid bone forms the support for the tongue and gives origin to muscles passing to the tongue and larynx. It also supports the thyroid cartilage (Fig. 104, 1).

_a_, body; _b_, _c_, _d_, _e_, cranial cornu; _f_, caudal cornu; _b_,
ceratohyal; _c_, epihyal; _d_, stylohyal; _e_, tympanohyal; _f_,
thyrohyal.]

It consists of a transverse bony bar, the =body= (Fig. 38, _a_) and of two =cornua= or horns attached to each end of the body.

The cranial cornu (lesser cornu of human anatomy) is the longer (Fig. 38, _b_-_e_). Each arises from the cranial face of the body at its lateral end, curves laterad, and then caudodorsad. It consists of four bony pieces movably united by cartilage.

The terminal piece is the =tympanohyal= (_e_); it is imbedded in the tympanic bulla just ventrad of the stylomastoid foramen. It is not therefore seen attached to the cornu after the latter has been separated from the skull. The other pieces become successively shorter toward the body, and are called =stylohyal= (_d_), =epihyal= (_c_), and =ceratohyal= (_b_).

The caudal cornua (_f_) (greater cornua of human anatomy) arise from the ends of the body. Each consists of a single piece of bone, the =thyrohyal= (_f_), which passes caudolaterad; its free end is united to a process of the thyroid cartilage (Fig. 104, 1).

=The Skull as a Whole.=--In the following description of the skull as a whole the mandible, hyoid, and ear-bones are not included.

The skull forms a bony box which contains the brain and is produced craniad into the =facial= portion which encloses the nasal cavity and forms the framework of the face.

In =dorsal view= (Fig. 39) the skull presents a smooth convex surface, broadest caudad, with the two =zygomatic arches= (_g_) curving out some distance laterally. The following bones are visible in dorsal view: the occipital (1), interparietal (2), parietals (3), temporals (4), frontals (5), malar or zygomatic bones (6), nasals (7), maxillaries (8), premaxillaries (9), and lachrymals (10).

1, occipital bone; 2, interparietal bone; 3, parietal bones; 4,
temporal; 5, frontal; 6, malar; 7, nasal; 8, maxillary; 9,
premaxillary; 10, lachrymal, _a_, lambdoidal ridge; _b_, external
occipital tubercle; _c_, sagittal crest; _d_, parietal eminence; _e_,
line which forms the dorsal boundary of the temporal fossa; _f_,
zygomatic process of the frontal; _g_, zygomatic arch; _h_, frontal
process of the malar; _i_, supraorbital arch; _j_, nares; _k_, foramen
incisivum or anterior palatine foramen; _l_, sphenopalatine foramen;
_m_, zygomatic process of the temporal; _n_, infraorbital foramen;
_o_, opening of lachrymal duct.]

The caudal boundary of the dorsal surface is marked by the prominent =lambdoidal ridge= (_a_) which passes from the middle cranioventrad along each side to the root of the zygomatic arch: it is borne by the occipital and temporal bones. From the middle of the lambdoidal ridge a second ridge, the =sagittal crest= (_c_), passes craniad in the middle line across the interparietal bone: it varies greatly in extent, reaching in a very old and muscular cat to the cranial border of the parietals, while in kittens it does not exist. The most prominent portions of the skull in this region, just craniad of the middle of the parietal bones, are known as the parietal tubercles or eminences (_d_). A faint curved line (_e_) runs from the cranial end of the sagittal crest craniolaterad to the base of the zygomatic process of the frontal: it marks the dorsal boundary of the origin of the temporal muscle, and may therefore be considered the dorsal boundary of the temporal fossa. This fossa extends from its dorsal boundary as far laterad and caudad as the lambdoidal ridge (_a_), and as far craniad as a line connecting the tip of the zygomatic process of the frontal (_f_) with the frontal process of the malar (_h_). The temporal muscle takes origin from its surface.

The middle portion of the dorsal surface is formed by the frontals (5). Each frontal presents laterally a prominent =zygomatic process= (_f_), extending ventrolaterad toward a corresponding (frontal) process (_h_) of the malar bone. These two processes mark the boundary between the orbital fossa (craniad) and the temporal fossa (caudad). Craniad of the zygomatic process of the frontal a sharp margin separates the dorsal surface of the skull from the wall of the orbital fossa: this is the =supraorbital arch= or margin (_i_).

The cranial portion of the dorsal surface is formed by the maxillary (8), nasal (7), and premaxillary bones (9). Just craniad of the nasals, bounded ventrad and craniad by the premaxillaries, appears the large opening of the nares (_j_), leading into the nasal cavity.

The =zygomatic arch= (_g_) is formed by the zygomatic process of the temporal (_m_) and the malar or zygomatic bone (6). Each presents near its middle a prominent dorsocaudally directed process, the frontal process (_h_) of the malar bone. The zygomatic arch forms the lateral boundary of the temporal and orbital fossæ, which are separated by a line connecting the frontal process of the malar (_h_) and the zygomatic process of the frontal (_f_).

A portion of the floor of the orbit and the opening of the lachrymal canal (_o_) may also be seen in dorsal view; they are described in connection with the lateral surface.

The =caudal surface= of the skull is formed largely by the occipital bone (Fig. 17), surrounding the foramen magnum (Fig. 17, _d_). At the sides of the foramen magnum are the two prominent curved occipital condyles (_e_) for articulation with the atlas. Craniolaterad of the condyles, separated from them by a deep notch, are the jugular processes (_f_) of the occipital, closely applied to the caudal ends of the tympanic bullæ.

Dorsad of the foramen magnum are faint indications of a median ridge running dorsad, the =external occipital crest= (Fig. 17, _i_); this rises at its junction with the lambdoidal ridge to form the prominent =external occipital tubercle= (Fig. 39, _b_). The dorsal and dorsolateral boundaries of the posterior surface are formed by the lambdoidal ridge (Fig. 17, _h_; Fig. 39, _a_).

The =lateral surface= of the skull (Fig. 40) is much more complicated than the dorsal and posterior surfaces. Caudally the occipital condyles (_a_) and external occipital crest (_b_) are visible; dorsocaudad the sagittal crest (_c_).

Extending from the caudal end of the sagittal crest the lambdoidal ridge (_d_) is seen passing ventrocraniad to the tympanic bulla, thence craniad to the root of the zygomatic arch. In the ventral part of the caudal region the tympanic bulla (_e_) is visible with the jugular process (_f_) of the occipital pressed close against its caudal end. Just craniad of the jugular process the mastoid process (_g_) of the temporal rests against the side of the bulla. Beneath the cranial edge of this process is the opening of the stylomastoid foramen (_h_) for the seventh nerve, while just ventrad of the foramen is the small pit (_i_) in the tympanic bulla for the reception of the tympanohyal bone. Craniad of the stylomastoid foramen is the large opening of the external auditory meatus (_j_), leading into the middle ear.

Immediately dorsocraniad of the external auditory meatus the zygomatic arch begins as the zygomatic process (_k_) of the temporal bone. On the cranial surface of the base of this process is the deep mandibular fossa (_l_) for the condyle of the mandible. This fossa is bounded caudally by the prominent postmandibular process (_m_).

1, occipital bone; 2, interparietal; 3, parietal; 4, temporal; 5, 5′,
frontal; 6, malar; 7, sphenoid; 8, palatine; 9, presphenoid; 10,
maxillary; 11, nasal; 12, premaxillary; 13, incisor teeth; 14, canine;
15, 16, 17, premolars; 18, molar. _a_, occipital condyle; _b_,
external occipital crest; _c_, sagittal crest; _d_, lambdoidal ridge;
_e_, tympanic bulla; _f_, jugular process; _g_, mastoid process; _h_,
stylo-mastoid foramen; _i_, pit for tympanohyal bone; _j_, external
auditory meatus; _k_, zygomatic process of temporal bone; _l_,
mandibular fossa; _m_, postmandibular process; _n_, zygomatic process
of the frontal; _o_, supraorbital margin; _p_, external pterygoid
fossa; _q_, sphenopalatine foramen; _r_, orbital fissure; _s_,
internal pterygoid fossa; _t_, hamulus; _u_, foramen ovale; _v_,
foramen rotundum; _w_, optic foramen; _x_, opening of lachrymal canal;
_y_, infraorbital foramen.]

All that portion of the lateral surface of the skull which lies craniodorsad of the lambdoidal ridge may be divided (excluding the zygomatic arch) into three main parts, the =temporal fossa=, the =orbital fossa=, and the =face=. The boundaries of the temporal fossa have been given. The orbital fossa is bounded externally by a prominent semicircular ridge formed chiefly by the zygomatic arch, the zygomatic process of the frontal (_n_), and the supraorbital arch (_o_) of the frontal, which may be traced to the cranial root of the zygomatic arch. The orbital fossa may be considered to end caudally and ventrally at the level of the optic foramen (_w_); ventrad of it are certain smaller fossæ. Immediately ventrad is the long =external pterygoid fossa= (_p_), from which arises part of the external pterygoid muscle. This fossa begins at the sphenopalatine foramen (_q_) and extends caudad to the orbital fissure (_r_); it is separated by a ridge from the orbital fossa. Caudoventrad of the external pterygoid fossa and separated from it by a sharp ridge is the small narrow =internal pterygoid= fossa (_s_), which extends ventrad without interruption on to the surface of the hamulus (_t_) and caudad to within two or three millimeters of the tympanic bulla. From it the internal pterygoid muscle takes origin. The hamulus (_t_) projects caudoventrad in this region, forming a prominent feature in a lateral view.

Four foramina leading into the cranial cavity are visible in a lateral view of the skull, craniad of the tympanic bulla. The one nearest the bulla is the =foramen ovale= (_u_) for the third division of the fifth nerve; next craniad of this is the =foramen rotundum= (_v_) for the second division of the fifth nerve. These two foramina pierce the alisphenoid: just craniad of them, between the alisphenoid and the orbitosphenoid, is the large =orbital fissure= (_r_) (foramen lacerum anterius), which transmits the third, fourth, and sixth cranial nerves and the first division of the fifth. Dorsocraniad of the orbital fissure is the =optic foramen= (_w_), for the =optic nerve=.

Ventrad of the cranial portion of the orbit is the large =sphenopalatine foramen= (_q_), for the nerves and arteries of the same name. Just craniad of this is the small caudal opening of the =posterior palatine canal=, which passes through the substance of the palatine bone and opens on its ventral surface near its cranial margin. Just dorsad of the cranial root of the zygomatic arch is the opening of the =lachrymal canal= (_x_), while the root of the arch is pierced by the large =infraorbital foramen= (_y_), which transmits the =infraorbital= nerves and artery from the orbit.

The teeth (13-18), implanted along the alveolar border of the maxillary and premaxillary, form a prominent feature in a lateral view: they are described in the account of the alimentary canal.

The =ventral surface= of the skull (Fig. 41) is very complex. It is separated by the orbits into a caudal and a cranial portion, united by a narrow median trough-like part. Laterad of this trough-like part are visible parts of the orbit and the zygomatic arches, which do not properly belong to the ventral surface and have already been described.

1, occipital bone; 2, temporal; 3, sphenoid; 4, presphenoid; 5,
frontal; 6, malar; 7, vomer; 8, palatine; 9, maxillary; 10,
premaxillary. _a_, foramen magnum; _b_, occipital condyles; _c_,
jugular process; _d_, tympanic bulla; _e_, mastoid process; _f_,
stylomastoid foramen; _g_, external auditory meatus; _h_, jugular
foramen; _i_, styliform process; _j_, groove for Eustachian tube; _k_,
foramen ovale; _l_, foramen rotundum; _m_, pterygoid process of
sphenoid; _n_, perpendicular plate of palatine; _o_, choanæ or
posterior nares; _p_, zygomatic arch; _q_, cranial end of posterior
palatine canal; _r_, palatine grooves; _s_, foramina incisiva or
anterior palatine foramina; _t_, opening of pterygoid canal.]

Caudally there appear in the ventral view the foramen magnum (_a_), occipital condyles (_b_), and jugular processes (_c_). In front of the jugular processes the two tympanic bullæ (_d_) form prominent features, with the mastoid process (_e_), the stylomastoid foramen (_f_), and the external auditory meatus (_g_) on their lateral surfaces. All these structures have been described. The tympanic bullæ (_d_) are placed with long axes directed craniomediad, so that they converge toward their cranial ends. At the caudomedial angle of each bulla is the large =jugular foramen= (_h_), for the ninth, tenth, and eleventh nerves. Opening into the mediocaudal margin of the jugular foramen is the smaller =hypoglossal= foramen, for the twelfth nerve.

The craniomedial end of the tympanic bulla projects craniad as the =styliform= process (_i_). Just laterad of this process is the opening (_j_) into the tympanic bulla by which the tuba auditiva or Eustachian tube passes into the middle ear. A faint groove for the tube passes craniomediad from this opening, on the surface of the sphenoid. Craniolaterad of the opening for the tuba auditiva is the =foramen ovale= (_k_); craniad of this the =foramen rotundum= (_l_) is faintly indicated. On the surface of the sphenoid just craniad of the styliform process of the bulla tympani is the minute opening of the =pterygoid canal= (_t_). The orbital fissure and optic foramen are not seen in the ventral view.

The middle region of the ventral surface is narrow: it is formed by a trough-like fossa which is bounded laterally by the pterygoid processes (_m_) of the sphenoid and the perpendicular plates of the palatines (_n_). Ventrad of this lies, in the natural condition, the soft palate, converting the fossa into the nasal portion of the pharynx or nasopharynx. Craniad this fossa is bounded by the free caudal edges of the palatines; beneath which the fossa communicates with the nasal cavity by the two choanæ (_o_). Laterad of this median fossa are visible in the ventral view parts of the temporal and orbital fossæ, bounded laterally by the zygomatic arches (_p_).

The cranial part of the ventral surface is a somewhat triangular plane area formed by the palatal portions of the palatines (8), maxillaries (9), and premaxillaries (10), which together constitute the hard palate (=palatum durum=). Laterad and craniad this area is bounded by the alveolar borders of the maxillaries and premaxillaries bearing the teeth. The hard palate is marked near the cranial border of the palatine bones with two or more foramina which form the cranial termination of the posterior palatine canal (_q_). Two faint grooves pass from these foramina a short distance craniad, gradually converging: these are known as the =palatine grooves= (_r_) (=sulci palatini=). Near the cranial end of the hard palate are two large openings close together near the middle line: these are the =foramina incisiva= (or =anterior palatine foramina=) (_s_).

_a_, foramen magnum; _b_, caudal end of hypoglossal canal; _c_,
jugular foramen; _d_, internal auditory meatus; _e_, tentorium,
forming the cranial boundary of the cerebellar fossa; _f_, dorsum
sellæ; _g_, sella turcica; _h_, anterior clinoid processes; _i_,
foramen ovale; _j_, foramen rotundum; _k_, orbital fissure; _l_, optic
foramen; _m_, chiasmatic groove; _n_, presphenoid bone; _o_,
cribriform plate; _p_, lamina perpendicularis of ethmoid; _q_,
labyrinths of ethmoid; _r_, nares; _s_, foramina incisiva or anterior
palatine foramina; _t_, infraorbital foramen; _u_, opening of the
lachrymal canal; _v_, caudal opening of posterior palatine canal; _w_,
sphenopalatine foramen; _x_, frontal process of the malar; _y_,
zygomatic process of the temporal; _z_, appendicular fossa, in the
petrous bone.]

=Cavities of the Skull= (Figs. 42 and 43).--The bones of the cranial portion of the skull enclose the =cranial cavity= for the brain; the facial bones enclose the =nasal cavity=, for the olfactory organ.

The =cranial= cavity is divisible into three principal fossæ: the =cerebellar= fossa (Fig. 43, _I_) caudad, for the cerebellum; the =cerebral= fossa (_II_) in the middle, for the cerebrum; the small =olfactory= fossa (_III_) at the cranial end for the olfactory bulb of the brain.

The =cerebellar= fossa (_I_) is bounded caudally by the occipital bone enclosing the foramen magnum (Fig. 42, _a_). Its ventral surface is formed by the basilar portion of the occipital and the petrous portions of the temporals; its lateral surface by the mastoid portions of the temporals and parts of the parietals and occipital. Its roof is formed by the parietals and interparietal. Craniad the cerebellar fossa is partly separated from the cerebral fossa by the tentorium (Fig. 42, _e_; Fig. 43, _f_) formed by the two parietals: this encloses a quadrangular opening by which the two fossæ communicate. The caudal, dorsal, and lateral walls of the cerebellar fossa are deeply marked by fossæ for the lobes of the cerebellum; the small =appendicular= fossa (Fig. 43, _e_), forming a deep indentation in the petrous bone near its dorsocaudal end, is particularly noticeable.

The following openings are found in the walls of the cerebellar fossa. Caudad is the large foramen magnum (Fig. 42, _a_) by which the brain-cavity communicates with the vertebral canal. Near the caudal margin of the foramen magnum, on its lateral side, just mediad of the dorsal end of the occipital condyle, is the caudal opening of the condyloid canal (Fig. 43, _a_) which passes craniad through the substance of the occipital bone to open just caudad of the petrous: it transmits a vein. The condyloid canal varies greatly in size in different specimens. A few millimeters craniad of the edge of the foramen magnum on the floor of the fossa is the small opening of the hypoglossal canal (Figs. 42 and 43, _b_), for the twelfth nerve. Just craniad of this, at the caudomedial border of the petrous, is the large jugular foramen (_c_). On the petrous itself, near the middle, is the internal, auditory meatus (_d_) divided into the dorsal =facial canal= for the seventh nerve, and a ventral passage for the eighth nerve. At the cranial end of the cerebellar fossa is the large opening bounded by the free edges of the tentorium.

The =cerebral= fossa forms much the largest part of the cranial cavity. It is bounded by the parietals (Fig. 43, 3′), squamous portions of the temporals (4), frontals (8), the sphenoid (5), and presphenoid (6). A slight rounded ridge on its lateral wall at about the position of the suture between the frontals and parietals separates a smaller cranial portion sometimes called the =anterior= fossa, from a larger caudal portion sometimes known as the =middle= fossa of the cranial cavity. The walls of the cerebral cavity are marked with numerous ridges and shallow furrows for the cerebral convolutions.

The floor of the cerebral cavity is bounded caudad by the prominent dorsum sellæ (Fig. 42, _f_; Fig. 43, _g_), just craniad of which is the rounded depression known as the sella turcica (Fig. 42, _g_; Fig. 43, _h_), for lodgment of the hypophysis. A number of foramina pierce the floor of the cavity in this region. Just ventrad of the cranial tip of the petrous portion of the temporal is the small =foramen lacerum= (medius). Craniad and laterad of this is a row of four foramina: the caudal one is the foramen ovale (Fig. 42, _i_); then come in order the foramen rotundum (_j_), the orbital fissure (_k_), and the optic foramen (Fig. 42, _l_; Fig. 43, _k_). The two optic foramina are connected by the shallow transverse =chiasmatic groove= (Fig. 42, _m_), for the optic chiasma. Another small foramen continues caudad from a groove on the floor of the orbital fissure; this opens on the ventral surface of the sphenoid, between the wing and the body of the bone. The groove and foramen constitute the =pterygoid canal=, which transmits a nerve,--the =nerve of the pterygoid canal=, or Vidian nerve.

The cranial cavity narrows at its cranial end to form the small =olfactory= fossa (Fig. 43, _III_) which lodges the olfactory bulbs. This is bounded by the frontals and the lamina cribrosa (Fig. 42, _o_) of the ethmoid; caudad it opens directly into the cerebral fossa. Numerous openings through the lamina cribrosa for the olfactory fibres connect the olfactory fossa with the nasal cavity. The roof of the fossa is marked by a prominent median crest from the united edges of the frontals.

The =nasal= cavity is almost completely filled by the ethmoid and vomer and the conchæ nasales. Its roof is formed by the nasal bones and portions of the frontals; its sides by the frontals, lachrymals, maxillaries, premaxillaries, and palatine bones; its floor by the horizontal plates of the palatines, maxillaries, and premaxillaries.

_I_, cerebellar fossa; _II_, cerebral fossa; _III_, olfactory fossa.
1, occipital bone; 2, interparietal; 3, 3′, parietal; 4, temporal (4,
squamous portion; 4′, petrous portion; 4″, tympanic portion); 5,
sphenoid; 6, presphenoid; 7, palatine; 8, frontal; 9, maxillary; 10,
premaxillary; 11, ethmoid; 12, nasal; 13, incisor teeth; 14, canine;
15, 16, 17, premolars; 18, molar. _a_, condyloid canal; _b_,
hypoglossal canal; _c_, jugular foramen; _d_, internal auditory
meatus; _e_, appendicular fossa; _f_, tentorium; _g_, dorsum sellæ;
_h_, sella turcica; _i_, hamular process; _j_, pterygoid process of
sphenoid; _k_, optic foramen; _l_, presphenoid sinus; _m_, _m′_,
frontal sinus; _n_, lamina perpendicularis of the ethmoid (broken at
cranial edge).]

The nasal cavity opens craniad by the large =nares= (Fig. 39, _j_; Fig. 42, _r_), which are bounded by the premaxillary and nasal bones. In the natural condition this opening is divided by a median cartilage which is continuous with the lamina perpendicularis (Fig. 43, _n_) of the ethmoid, thus forming a partition which divides the nasal cavity into two separate halves. From the floor of the cranial part of the cavity rises a ridge formed of the nasal crests of the maxillaries and premaxillaries, and the cranial portion of the vomer. Farther caudad the vomer spreads out in a horizontal plane and separates from the floor of the cavity, so that the nasal cavity is thereby divided by a horizontal partition into dorsal and ventral portions. The ventral portion is small, forming the inferior meatus of the nose; it ends caudally at the =choanæ= (posterior nares, Fig. 41, _o_) which lead into the nasopharynx. That portion of the nasal cavity lying dorsad of the vomer is almost completely filled by the ethmoid and the conchæ nasales, superior and inferior. It is bounded caudally by the lamina cribrosa of the ethmoid (Fig. 42, _o_). The nasal cavity communicates with the cranial cavity by the foramina for the olfactory fibres in the lamina cribrosa; with the nasopharynx by the choanæ; with the exterior of the body by the nares; with the mouth-cavity by the foramina incisiva or anterior palatine foramina (Fig. 42, _s_); with the orbit by the sphenopalatine foramen and the nasolachrymal canal. It communicates directly also with the frontal sinuses (Fig. 43, _m_, _m′_), the sphenoidal sinuses (Fig. 43, _l_), and with the cells of the labyrinths of the ethmoid.

JOINTS AND LIGAMENTS OF THE SKULL.

=Sutures of the Skull.=--The bones of the skull join each other by means of immovable articulations known as sutures. These sutures are designated by combining the names of the bones between which they are situated: as, =sphenofrontal= suture (=sutura sphenofrontalis=), between the sphenoid and frontal; =nasomaxillary= suture (=sutura nasomaxillaris=), between the nasal and maxillary bones. When a suture joins the two corresponding bones of opposite sides the prefix =inter= is used, as the =intermaxillary= suture (=sutura intermaxillaris=) between the maxillaries. The sutures bounding the parietals have, however, received special names not derived in this manner. The suture caudad of the parietals, separating them from the occipital and interparietal, is known as the =lambdoidal= suture; that between the two parietals is the =sagittal= suture; that separating the parietals and squamous portions of the temporals is the =squamous= suture; that between the parietals and frontals is the =coronal= suture. The suture separating the two frontals also is known as the =frontal= suture, in place of interfrontal.

=Articulations of the Mandible.=--In man the two halves of the mandible are united craniad, so as to form a single bone. In the cat the two halves are separate, but articulate closely at the =symphysis menti= by a thin interarticular cartilage.

The articulation of the mandible at the mandibular fossa of the temporal is covered with a close =articular capsule=. The mandibular fossa is lined with cartilage. A slender ligament passes from the angular process of the mandible caudad to the external auditory meatus, being attached to the latter about 8 millimeters from its medial end. This is the =stylomandibular ligament=.