The maxillae are the largest bones of the face, excepting the mandible, and by their union form the whole of the upper jaw (fig. 314). Each assists in completing the roof of the mouth, the floor and lateral wall of the nasal cavity, and the floor of the orbit; it also enters into the formation of the infratemporal and pterygopalatine fossa;, and the inferior orbital and pterygomaxillary fissures.
Each maxilla consists of a body and four processes- zygomatic, frontal, alveolar, and palatine.
The body of the maxilla is somewhat pyramidal in shape. It has four surfaces–anterior, posterior; orbital, and nasal-and encloses a large cavity, termed the maxillary sinus.
The posterior surface (infratemporal surface) (fig. 383) is convex, directed backwards and laterally and forms the anterior wall of the infratemporal fossa. It is separated from the anterior surface by the zygomatic process and by a ridge which runs upwards to that process from the socket of the first molar tooth. It is pierced near its centre by the apertures of two or three dental canals (alveolar canals), which transmit the posterior superior dental (alveolar) vessels and nerves. At the lower part of this surface there is a round eminence, termed the maxillary tuberosity, which is rough for articulation with the tubercle (pyramidal process) of the palatine bone (fig. 383); it gives origin to a few fibres of the medial pterygoid muscle and, in some cases, articulates with the lateral pterygoid plate of the sphenoid bone. Above this a smooth surface forms the anterior boundary of the pterygopalatine fossa and is grooved for the maxillary nerve ; the groove for this nerve is directed laterally and slightly upwards and is continuous with the infra-orbital groove on the orbital surface.
The nasal surface (fig. 385) presents in its upper and posterior part a, large, irregular opening, termed the maxillary hiatus, which leads into the maxillary sinus. At the upper border of this aperture there are some broken air-sinuses, which, in the articulated skull, are closed by the ethmoid and lacrimal bones. Below the maxillary hiatus a smooth concave surface forms part of the inferior meatus of the nasal cavity, and behind it there is a rough surface for articulation with the perpendicular plate of the palatine bone ; this rough surface is traversed by a groove, which. begins near the middle of the posterior border, runs obliquely downwards and forwards, and is converted into the greater palatine canal (pterygopalatine canal) by the perpendicular plate of the palatine bone. In front of the maxillary hiatus a deep groove, which is continuous above with the lacrimal sulcus, constitutes about two-thirds of the circumference of the nasolacrimal canal, the remaining one-third being formed by the descending part of the lacrimal bone and the lacrimal process of the inferior nasal concha (fig. 375); this canal opens into the inferior meatus of the nose (fig. 371) and transmits the nasolacrimal duct. More anteriorly the bone is marked by an oblique ridge, termed the conchal crest for articulation with the inferior nasal concha. The shallow concavity below- this ridge forms part of the inferior meatus of the nose, and the surface above the ridge part of the atrium of the middle meatus.
The maxillary sinus (figs. 385, 386) is a large pyramidal cavity within the body of the maxilla. Its walls are thin and correspond to the nasal, orbital, anterior and posterior surfaces of the body of the bone. Its apex, directed laterally, is formed by the zygomatic process; its base, or nasal wall, directed medially, is formed by the lateral wall of the nose and presents the maxillary hiatus in the disarticulated bone. In the articulated skull this aperture is much reduced in size by the following bones : the uncinate process of the ethmoid and the descending part of the lacrimal bone above, the maxillary process of the inferior nasal concha below, and the perpendicular plate of the palatine bone behind (figs. 371, 386). The maxillary sinus communicates with the middle meatus of the nose, generally by two small apertures, one of which is usually closed in the recent state by mucous membrane. The posterior wall is pierced by the dental canals (alveolar canals), which transmit the posterior superior dental (alveolar) vessels and nerves to the molar teeth; these canals occasionally project as ridges into the maxillary sinus. The floor is formed by the alveolar process of the maxilla, and its lowest past is usually about 1.25 cm. below the level of the floor of the nasal cavity. In a large proportion of bones, radiating septa of varying sizes spring from the floor of the sinus in the intervals between adjacent teeth ; in some cases the floor is perforated by the fangs of the molar teeth.* The infra-orbital canal usually projects into the sinus as a well-marked ridge extending from the roof to the anterior wall. The size of the cavity varies in different skulls, and even on the two sides of the same skull.*
* The number of teeth whose roots are in relation with the floor of the maxillary sinus is variable. The sinus may extend so as to be in relation to all the teeth of the true maxilla, from the canine to the third molar.-(Salter)
* Logan Turner (op. cit.) gives the following measurements for an adult sinus of average size : vertical height opposite first molar tooth, 3-5 cm. ; transverse breadth, 2.5 cm. ; and anteroposterior depth, 3.2 cm.
The zygomatic process of the maxilla is a. rough pyramidal eminence, situated at the angle of separation of the anterior. posterior, and orbital surfaces. ha, front it forms part of the anterior surface of the body of the bone ; behind, it is concave, and continuous with the posterior surface ; above, it is rough and serrated for articulation with the zygomatic bone; below, it presents a prominent arched border, which separates the anterior from the posterior surface.
The frontal process of the maxilla projects upwards and backwards between the nasal and lacrimal bones (figs. 333, 386). Its lateral surface (fig. 383) is divided by a vertical ridge, termed the lacrimal crest, which gives attachment to the medial palpebral ligament and is continuous below with the infra-orbital margin. At the junction of the crest with the orbital surface is a small tubercle, which serves as a guide to the position of the lacrimal sac. The part in front of the lacrimal crest is smooth and merges below with the anterior surface of the body; it gives attachment to a portion of the orbicularis oculi and to the levator labii superioris alaque nasi (angular head of the quadratus labii superioris). Behind the lacrimal crest a vertical groove unites with the groove on the lacrimal bone to complete the lacrimal groove for the lodgment of the lacrimal sac.
The alveolar process of the maxilla, thick and arched, is broader behind than in front, and excavated into sockets (alveoli) for the reception of the roots of the teeth. These cavities are eight in number and vary in size and depth according to the teeth they contain. That for the canine tooth is the deepest; those for the molars are the widest, and are sub. divided into three minor sockets by septa; those for the incisors and the second premolar are single; that for the first premolar is sometimes divided into two. The buccinator muscle arises from the outer surface of this process, as far forward as the first molar tooth. When the maxillae are articulated with each other, their alveolar processes together form the alveolar arch.
The palatine process of the maxilla, thick and strong, is horizontal and projects medially from the lowest part of the nasal surface of the bone. It forms a, considerable part of the floor of the nose and the roof of the mouth, and is much thicker in front than behind. Its inferior surface (fig. 387), concave, rough. and uneven, forms, with the palatine process of the opposite bone, the anterior three-fourths of the bony palate. It is perforated by numerous foramina for the passage of the nutrient vessels and presents depressions for the lodgement of the palatine glands; it is channeled at the posterior part of its lateral border by two grooves, which lodge the greater palatine vessels and nerve (descending palatine vessels and the anterior palatine nerve). When the two maxillae are articulated, a funnel-shape depression, termed the incisive fossa (incisive foramen) is seen in the median plane, immediately behind the incisor teeth. In this opening the orifices of two lateral canals are visible; they are named the incisive canals ; each leads upwards into the corresponding nasal cavity and transmits the terminal branch of the greater palatine artery and the long sphenopalatine (nasopalatine) nerve. Occasionally there are two additional apertures in the median plane; they are termed the anterior and posterior incisive foramina, and, when present, transmit the long sphenopalatine (nasopalatine) nerves, the left passing through the anterior, and the right through the posterior foramen. On the under surface of the palatine process, a delicate suture, well seen in young skulls, may sometimes be noticed extending laterally and forwards from the incisive fossa to the interval between the lateral incisor and the canine teeth. The small part in front of this suture constitutes the os incisivnm (premaxilla), which in most vertebrates forms an independent bone: it includes the whole thickness of the alveolus, the corresponding part of the floor of the nose and the anterior nasal spine and contains the sockets of the incisor teeth. The upper surface of the palatine process is concave from side to side, smooth, and forms the greater part of the floor of the nasal cavity; close to the anterior part of its medial margin the bone is pierced by the upper orifice of the incisive canal. The lateral border of the process is fused with the rest of the bone. The medial border, thicker in front than behind, is raised into a ridge, termed the nasal crest, which, with the corresponding ridge of the opposite bone, forms a groove for the reception of the vomer. The front part of this ridge rises to a considerable height and is sometimes named the incisor crest (fig. 385) ; it is prolonged forwards into a sharp process, which, with the similar process of the opposite bone, forms the anterior nasal spine. The posterior border is serrated for articulation with the horizontal plate of the palatine bone.
At birth the transverse and anteroposterior diameters of the maxilla are each greater than the vertical. The frontal process is well-marked, but the body of the bone consists of little more than the alveolar process, the tooth-sockets reaching almost to the floor of the orbit. The maxillary sinus is seen as a furrow on the lateral wall of the nose. In the adult the vertical diameter is the greatest, owing to the development of the alveolar process and the increase in size of the sinus. In old age the bone reverts in some measure to the infantile condition; its height is diminished and, after the loss of the teeth, the alveolar process is absorbed, and the lower part of the bone contracted and reduced in thickness.
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