Maxilla

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.

Figure 383
The anterior surface (fig. 383) is directed forwards and laterally. Its lower part displays a number of slight elevations, which correspond to the positions of the roots of the upper teeth. Just above those of the incisor teeth there is a slight depression, named the incisive fossa, which gives origin to the depressor septi; to the alveolar border below the fossa a slip of the orbicularis oris is attached ; above and lateral to the fossa, the compressor nasis muscle arises. Lateral to the incisive fossa there is a larger and deeper depression, named the canine fossa ; it is separated from the incisive fossa, by the canine eminence, which corresponds to the socket of the canine tooth; the fossa gives origin to the levator anguli oris (caninus) muscle. Above the canine fossa the infra-orbital foramen marks the anterior end of the infra,-orbital canal ; it transmits the infra-orbital vessels and nerve. Above the foramen a sharp border marks the junction of the anterior and orbital surfaces. This border forms a small part of the circumference of the orbital opening, and gives origin to the levator labii superioris (infra.-orbital head of the quadratus labii superioris). Medially, the anterior surface is limited by a deep concavity, termed the nasal notch; the margin of the notch gives attachment to the dilator naris and ends below in a pointed process which, with the corresponding process of the opposite maxilla, forms the anterior nasal spine.

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.

Figure 384
The orbital surface (fig. 383) is smooth aid triangular, and forms the greater part of the floor of the orbit. Its medial border presents anteriorly a notch, termed the lacrimal notch, behind which it articulates from before backwards with the lacrimal bone, the orbital plate (lamina papyracea) of the ethmoid, and the orbital process of the palatine bone (fig, 386). Its posterior border is smooth and rounded; it forms the greater part of the anterior margin of the inferior orbital fissure, and its central part is notched by the commencement of the infraorbital groove. The anterior border forms a small part of the circumference of the orbital opening and is continuous medially with the lacrimal crest on the frontal process. The infra-orbital groove, for the passage of the infra-orbital vessels and nerve, begins at the middle of the posterior border, where it is continuous with the groove near the upper edge of the posterior surface; it passes forwards and ends in the infra-orbital canal, which opens on the anterior surface of the bone just below the infra-orbital margin. At the medial and front part of the orbital surface, and lateral to the lacrimal groove, a small depression gives origin to the inferior oblique muscle of the eyeball.

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.

Figure 385
Applied Anatomy.-The extreme thinness of the walls of this cavity affords an explanation of the fact that a tumor growing from the maxillary sinus and encroaching upon the adjacent parts may push up the floor the orbit, and displace the eyeball ; may project into the nose ; may protrude forwards on to the cheek ; or may make its way backwards into the infratemporal fossa, or downwards into the mouth.

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.

Figure 386
The medial surface of the frontal process (fig. 385) forms a portion of the lateral wall of the nasal cavity. A rough, uneven area at its upper part articulates with the ethmoid bone and closes the anterior ethmoidal sinuses. Below this rough area there is an oblique ridge, termed the ethmoidal crest, the posterior part of which articulates with the middle nasal concha, while the anterior part is termed the agger nasi ; the ethmoidal crest forms the upper limit of the atrium of the middle meatus of the nose. The upper end of the frontal process articulates with the nasal notch of the frontal bone, the anterior border with the nasal bone, and the posterior border with the lacrimal bone.

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.

Figure 387
Ossification.-The maxilla, is mainly developed in membrane. Mall* and Fawcett have shown that it is ossified from two principal centres, one for the maxilla proper and one for the os incisivum (premaxilla). These centres appear about the end of the sixth week of fetal life, that for the maxilla proper above the canine tooth-germ, and that for the os incisivum above the incisor tooth-germs ; the latter centre grows upwards and forms the anterior part of the frontal process. These two centres unite at the end of the second or early in the third month, but the suture between them (fig. 389) may persist on the palate until nearly middle life. At a later stage an additional centre (prevomerine or paraseptal centre) appears for the os incisivum on the medial side of the corresponding paraseptal cartilage. The maxillary sinus appears as a shallow groove (fig. 390) on the nasal surface of the bone about the fourth month of fetal life, but does not reach its full size until after the second dentition. The infra-orbital vessels and nerve lie for a time in an open groove in the floor of the orbit; the anterior part of this groove is converted into the infra-orbital canal by a lamina of bone which grows from the lateral side of the groove.

Figure 388
Figure 389
Figure 390
CHANGES PRODUCED IN THE MAXILLA BY AGE

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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