The =maxillæ= are the principal bones of the upper jaw and carry the
upper cheek teeth. They are situated on the lateral aspect of the face,
and articulate with almost all of the facial bones and the frontal and
temporal also. For description each may be divided into a body and two
processes.
The =body= (Corpus maxillæ) presents two surfaces, two borders, and two
extremities. The =external= or =facial surface= (Facies lateralis) is
somewhat concave in front and convex behind. On its posterior part is a
horizontal ridge, the =facial= or =zygomatic crest= (Crista facialis);
in a skull of medium size its anterior end is about an inch and a half
(3 to 4 cm.) above the third or fourth cheek tooth, and it is continued
behind by a corresponding ridge on the malar bone. About two inches (5
cm.) above and a little in front of the anterior end of the crest is the
=infraorbital foramen= (Foramen infraorbitale); this is the external
opening of the infraorbital canal.
In the young horse the anterior part of the surface is convex over the
embedded parts of the teeth. As the latter are extruded the surface
flattens and becomes concave in old subjects.
The =internal= or =nasal surface= (Facies nasalis) is concave
dorso-ventrally; it forms the greater part of the lateral wall of the
nasal cavity. Its upper part is crossed obliquely forward and downward
by the shallow =lacrimal groove= (Sulcus lacrimalis), which contains the
naso-lacrimal duct; in the adult the posterior part of the groove is
converted into a canal, which is continuous with that on the inner
surface of the lacrimal bone. Below the groove is the =inferior turbinal
crest= (Crista conchalis inferior), to which the inferior turbinal bone
is attached. Lower down and parallel with the turbinal crest is the
=palatine process=, which projects inward like a shelf. Behind this the
surface is rough for articulation with the palate bone; this area is
crossed by a groove which concurs with a furrow on the palate bone in
the formation of the =palatine canal=. The posterior part of the bone is
excavated to form part of the maxillary sinus.
The =superior border= is irregular and scaly. Its anterior part is
grooved and its posterior part beveled for articulation with the nasal
process of the premaxilla and the nasal and lacrimal bones.
The =inferior= or =alveolar border= (Processus alveolaris) is thick, and
presents six large cavities, the =alveoli=, for the cheek teeth. The
alveoli are separated by transverse =interalveolar septa=. There is
often a small alveolus for the first premolar (“wolf tooth”) close to
the first large one. Further forward the border is narrow and forms part
of the =interdental= or =interalveolar space= (Margo interalveolaris).
Behind the last alveolus is a rough area, the =alveolar tuberosity=.
The =anterior extremity= is pointed. It joins the premaxilla, and forms
with it the alveolus for the canine tooth.
The =posterior extremity= forms a rounded prominence, the =maxillary
tuberosity= (Tuber maxillare). Internal to the tuberosity is a deep
cavity, the maxillary hiatus, in which are three foramina. The upper
one, the =maxillary foramen=, leads into the infraorbital canal. The
lower one, the =posterior palatine foramen=, is the entrance to the
palatine canal. The =sphenopalatine foramen= perforates the inner wall
of the hiatus and opens into the nasal cavity.
The =zygomatic= or =temporal process= (Processus temporalis) projects
backward above and external to the tuberosity; it is overlapped by the
corresponding part of the malar and also articulates with the zygomatic
process of the temporal. A small curved plate extends inward from it and
joins the frontal and palate bones, forming part of the floor of the
orbit.
The =palatine process= (Processus palatinus) is a plate which projects
horizontally inward from the lower part of the inner surface of the
body. It forms the greater part of the basis of the hard palate. Its
=superior= or =nasal surface= is smooth and concave transversely; on its
anterior part, close to the inner border, is a shallow groove in which
the organ of Jacobson is situated. The inferior or =palatine surface= is
slightly concave from side to side, and presents along its outer part
the =palatine groove= (Sulcus palatinus). The groove is a continuation
of the palatine canal, and contains the palatine artery and nerve. The
=internal border= unites with its fellow to form the median palatine
suture; its nasal aspect bears the nasal crest, which forms, with that
of the opposite process, a groove for the vomer. The =posterior border=
unites with the horizontal part of the palate bone at the transverse
palatine suture.
[Illustration:
FIG. 31.—UPPER JAW OF HORSE ABOUT FOUR AND A HALF YEARS OLD, VENTRAL
VIEW.
1, 1, Posterior nares; 2, vomer; 3, horizontal part of palate bone; 4,
anterior palatine foramen; 5, palatine groove; 6, transverse
palatine suture; 7, median palatine suture; 8, palate process of
maxilla; 9, palate process of premaxilla; 10, foramen incisivum; 11,
malar bone; 12, maxilla; 13, anterior end of facial crest; 14,
interalveolar space; I. 1–3, incisor teeth; _C_, canine tooth; _P1_,
first premolar or “wolf” tooth.
]
The =infraorbital= or =superior dental canal= extends almost
horizontally from the maxillary foramen to the infraorbital foramen. It
is placed at the upper edge of the inner plate of the maxilla, and
traverses the maxillary sinus. Near the infraorbital foramen it gives
off a small canal (Canalis alveolaris incisivus) which lies above the
roots of the premolars and extends also into the premaxilla, carrying
vessels and nerves to the teeth there.
=Development.=—The maxilla ossifies in membrane below and external to
the cartilaginous nasal capsule. It has one chief center and a
supplementary one in the region of the deciduous canine tooth (Martin).