_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).
[Illustration: FIG. 38.--HYOID BONE, DORSAL VIEW.
_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).
[Illustration: FIG. 39.--SKULL, DORSAL SURFACE.
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_).
[Illustration: FIG. 40.--SKULL, SIDE VIEW.
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.
[Illustration: FIG. 41.--SKULL, VENTRAL VIEW.
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_).
[Illustration: FIG. 42.--SKULL, WITH DORSAL SURFACE REMOVED, SHOWING
THE CRANIAL AND NASAL CAVITIES.
_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.
[Illustration: FIG. 43.--SKULL, MEDIAN LONGITUDINAL SECTION, SHOWING
THE CAVITIES.
_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.