The orbiter surface of the bone is the small est; it is quadrilateral, smooth, and slightly concave, with an inclination from above down wards and from within outwards ; it forms the greater part of the floor of the orbit. Along the middle of its posterior half runs, in a direc tion forwards and outwards, the infra-orbitar groove, which anteriorly becomes a complete canal (the infra-orbitar canal), and finally divides into an internal or larger canal, which terminates at the infra-orbitar hole in the canine fossa, and into an external or small conduit, which runs in the anterior wall of the antrum, and conveys the superior anterior den tal nerves to the incisor and canine teeth ; this outer subdivision of the canal presents several varieties in different individuals. The orbitar surface (or plate) has four borders: 1. The posterior, which, free and notched in the mid dle by the commencement of the infra-orbitar canal, forms with the orbitar plate of the sphe noid and palate bones the inferior orbitar or the spheno-maxillary fissure. 2. The internol, which articulates from behind forwards succes sively with the palate, the ethmoid, and the lachrymal bones. 3. The anterior, short and smooth, separates the orbitar from the facial surfaces of the bone; at its inner extremity is the nasal process already described. 4. The external is united to the malar bone; on the outer side of this border is a rough triangular projecting surface (the nzalar process) which receives the os malw, and which forms an angle of union between the anterior, posterior, and superior surfaces of the upper maxillary bone.
The internal or naso-palatine surface is di vided along the anterior three-fourths into two unequal parts by an horizontal plate of bone (the palatine process): above this is the nasal portion forming the upper three-fourths of this surface, and below it, is the palatine part which forms the remaining fourth. The palatine process forms the anterior three fourths of the floor of the nose, and roof of the mouth; it presents a smooth upper surface, concave transversely, and nearly flat in the op posite direction : it is broad behind and narrow in front, where there is placed the orifice of the anterior palatine caned, which takes a direction downwards, forwards, and inwards, unites with the corresponding canal in the opposite bone at the median plane, and forms a common canal (the canalis incirivus), which opens below by a hole (the foramen ineisivunt) on the roof of the mouth, immediately behind the middle incisor teeth. The anterior palatine canals and the incisive canal, which are often included to gether under a common name, form a tube re sembling the letter Y, being bifid above and single below. The inferior surface of the pa latine process is rough and concave, and forms the anterior and larger part of the roof of the mouth ; its internal border is long and rough, thick in front, narrow behind, and united with the corresponding border of the opposite hone forms the maxillary suture : this border is sur mounted by a half-furrow which, with that of its fellow bone, forms a groove for the reception of a part of the vomer. The posterior border is short
and cut obliquely at the expense of the upper surface ; it supports the anterior margin of the horizontal part of the palate-bone. The pala tine division of the internal surface of the upper maxillary bone is narrow, and forms part of the arched roof of the mouth ; along its junc tion with the palatine process is a broad shal low groove for lodging the posterior palatino nerves and vessels. The nasal portion of the internal surface is placed above the palatine process, and is lined on its anterior three-fourths by the pituitary membrane. Tracing this sur face from before backwards we observe, 1. the lower aperture of the naso-lachrymal canal, situate just behind the inferior turbinated crest of the nasal process ; 2. posterior to this, the orifice of the maxillary sinus, or antrum of Ilighmore, which in the sepaiated bone is a large opening, but is contracted in the united face by the lachrymal, the ethmoid, the palate, and the inferior turbinated bones, which are attached around its margin. Above this aper ture are seen some cells which unite with those of the ethmoid, and its lower edge presents a fissure in which is received the maxillary pro cess of the palate-bone. Below the inferior turbinated crest, the naso-lachrymal canal and the orifice of the antrum, the bone is concave and smooth, and forms a part of the inferior meatus of the nose; behind this smooth surface and the orifice of the antrum, the bone is rough for the attachment of the vertical plate of the os palati, and it presents a groove, which, descending obliquely forwards to the palatine division of this surface, forms a part of the posterior palatine canal.
The maxillary sinus (sinus maxillaris, antrum Ilighmori ; Germ. die Oberkieferhiihle) oc cupies in the adult the whole body of the bone : its form is triangular, with the base directed internally towards the orifice which has been already described, and the apex out wards towards the malar process. Its superior wall is formed by the orbitar plate; the pos terior corresponds to the maxillary tuberosity ; and the anterior to the canine fossa. All these walls present ridges or crests, which lodge canals for the passage of nerves. The posterior and anterior walls contain the su perior, anterior, dental canals, which lodge nerves of the same name. The tipper wall contains the infra-orbitar groove and canal, which gives passage to the upper maxillary nerve.