_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.
[Illustration: FIG. 33.--PALATINE BONE, DORSAL VIEW.
_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.
[Illustration: FIG. 34.--LACHRYMAL BONE OF LEFT SIDE, EXTERNAL
SURFACE.