_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.
[Illustration: FIG. 29.--PREMAXILLARY BONE, OBLIQUELY CRANIOLATERAL
ASPECT.
_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.
[Illustration: FIG. 30.--NASAL BONE, DORSAL VIEW.]
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.
[Illustration: FIG. 31.--ETHMOID AND VOMER, SIDE VIEW.