Part 12

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.

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).