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5b. 2. The Maxillæ (Upper Jaw)
FIG. 157 - Left maxilla. Outer surface. The maxillæ are the largest bones of the face, excepting the mandible, and form, by their union, the whole of the upper jaw. Each assists in forming the boundaries of three cavities, viz., the roof of the mouth, the floor and lateral wall of the nose and the floor of the orbit; it also enters into the formation of two fossæ, the infratemporal and pterygopalatine, and two fissures, the inferior orbital and pterygomaxillary. Each bone consists of a body and four processes—zygomatic, frontal, alveolar, and palatine. The Body (corpus maxillæ).—The body is somewhat pyramidal in shape, and contains a large cavity, the maxillary sinus (antrum of Highmore). It has four surfaces—an anterior, a posterior or infratemporal, a superior or orbital, and a medial or nasal. Surfaces.—The anterior surface (Fig. 157) is directed forward and lateralward. It presents at its lower part a series of eminences corresponding to the positions of the roots of the teeth. Just above those of the incisor teeth is a depression, the incisive fossa, which gives origin to the Depressor alæ nasi; to the alveolar border below the fossa is attached a slip of the Orbicularis oris; above and a little lateral to it, the Nasalis arises. Lateral to the incisive fossa is another depression, the canine fossa; it is larger and deeper than the incisive fossa, and is separated from it by a vertical ridge, the canine eminence, corresponding to the socket of the canine tooth; the canine fossa gives origin to the Caninus. Above the fossa is the infraorbital foramen, the end of the infraorbital canal; it transmits the infraorbital vessels and nerve. Above the foramen is the margin of the orbit, which affords attachment to part of the Quadratus labii superioris. Medially, the anterior surface is limited by a deep concavity, the nasal notch, the margin of which gives attachment to the Dilatator naris posterior and ends below in a pointed process, which with its fellow of the opposite side forms the anterior nasal spine. The infratemporal surface (Fig. 157) is convex, directed backward and lateralward, and forms part of the infratemporal fossa. It is separated from the anterior surface by the zygomatic process and by a strong ridge, extending upward from the socket of the first molar tooth. It is pierced about its center by the apertures of the alveolar canals, which transmit the posterior superior alveolar vessels and nerves. At the lower part of this surface is a rounded eminence, the maxillary tuberosity, especially prominent after the growth of the wisdom tooth; it is rough on its lateral side for articulation with the pyramidal process of the palatine bone and in some cases articulates with the lateral pterygoid plate of the sphenoid. It gives origin to a few fibers of the Pterygoideus internus. Immediately above this is a smooth surface, which forms the anterior boundary of the pterygopalatine fossa, and presents a groove, for the maxillary nerve; this groove is directed lateralward and slightly upward, and is continuous with the infraorbital groove on the orbital surface. The orbital surface (Fig. 157) is smooth and triangular, and forms the greater part of the floor of the orbit. It is bounded medially by an irregular margin which in front presents a notch, the lacrimal notch; behind this notch the margin articulates with the lacrimal, the lamina papyracea of the ethmoid and the orbital process of the palatine. It is bounded behind by a smooth rounded edge which forms the anterior margin of the inferior orbital fissure, and sometimes articulates at its lateral extremity with the orbital surface of the great wing of the sphenoid. 
FIG. 158 - Left maxilla. Nasal surface. It is limited in front by part of the circumference of the orbit, which is continuous medially with the frontal process, and laterally with the zyogmatic process. Near the middle of the posterior part of the orbital surface is the infraorbital groove, for the passage of the infraorbital vessels and nerve. The groove begins at the middle of the posterior border, where it is continuous with that near the upper edge of the infratemporal surface, and, passing forward, ends in a canal, which subdivides into two branches. One of the canals, the infraorbital canal, opens just below the margin of the orbit; the other, which is smaller, runs downward in the substance of the anterior wall of the maxillary sinus, and transmits the anterior superior alveolar vessels and nerve to the front teeth of the maxilla. From the back part of the infraorbital canal, a second small canal is sometimes given off; it runs downward in the lateral wall of the sinus, and conveys the middle alveolar nerve to the premolar teeth. At the medial and forepart of the orbital surface just lateral to the lacrimal groove, is a depression, which gives origin to the Obliquus oculi inferior. The nasal surface (Fig. 158) presents a large, irregular opening leading into the maxillary sinus. At the upper border of this aperture are some broken air cells, which, in the articulated skull, are closed in by the ethmoid and lacrimal bones. Below the aperture is a smooth concavity which forms part of the inferior meatus of the nasal cavity, and behind it is a rough surface for articulation with the perpendicular part of the palatine bone; this surface is traversed by a groove, commencing near the middle of the posterior border and running obliquely downward and forward; the groove is converted into a canal, the pterygopalatine canal, by the palatine bone. In front of the opening of the sinus is a deep groove, the lacrimal groove, which is converted into the nasolacrimal canal, by the lacrimal bone and inferior nasal concha; this canal opens into the inferior meatus of the nose and transmits the nasolacrimal duct. More anteriorly is an oblique ridge, the conchal crest, for articulation with the inferior nasal concha. The shallow concavity above this ridge forms part of the atrium of the middle meatus of the nose, and that below it, part of the inferior meatus. 
FIG. 159 - Left maxillary sinus opened from the exterior. The Maxillary Sinus or Antrum of Highmore (sinus maxillaris).—The maxillary sinus is a large pyramidal cavity, within the body of the maxilla: its apex, directed lateralward, is formed by the zygomatic process; its base, directed medialward, by the lateral wall of the nose. Its walls are everywhere exceedingly thin, and correspond to the nasal orbital, anterior, and infratemporal surfaces of the body of the bone. Its nasal wall, or base, presents, in the disarticulated bone, a large, irregular aperture, communicating with the nasal cavity. In the articulated skull this aperture is much reduced in size by the following bones: the uncinate process of the ethmoid above, the ethmoidal process of the inferior nasal concha below, the vertical part of the palatine behind, and a small part of the lacrimal above and in front (Figs. 158, 159); the sinus communicates with the middle meatus of the nose, generally by two small apertures left between the above-mentioned bones. In the fresh state, usually only one small opening exists, near the upper part of the cavity; the other is closed by mucous membrane. On the posterior wall are the alveolar canals, transmitting the posterior superior alveolar vessels and nerves to the molar teeth. The floor is formed by the alveolar process of the maxilla, and, if the sinus be of an average size, is on a level with the floor of the nose; if the sinus be large it reaches below this level. Projecting into the floor of the antrum are several conical processes, corresponding to the roots of the first and second molar teeth; (*38 in some cases the floor is perforated by the fangs of the teeth. The infraorbital canal usually projects into the cavity as a well-marked ridge extending from the roof to the anterior wall; additional ridges are sometimes seen in the posterior wall of the cavity, and are caused by the alveolar canals. The size of the cavity varies in different skulls, and even on the two sides of the same skull. (*39 The Zygomatic Process (processus zygomaticus; malar process).—The zygomatic process is a rough triangular eminence, situated at the angle of separation of the anterior, zygomatic, and orbital surfaces. In front it forms part of the anterior surface; behind, it is concave, and forms part of the infratemporal fossa; above, it is rough and serrated for articulation with the zygomatic bone; while below, it presents the prominent arched border which marks the division between the anterior and infratemporal surfaces. The Frontal Process (processus frontalis; nasal process).—The frontal process is a strong plate, which projects upward, medialward, and backward, by the side of the nose, forming part of its lateral boundary. Its lateral surface is smooth, continuous with the anterior surface of the body, and gives attachment to the Quadratus labii superioris, the Orbicularis oculi, and the medial palpebral ligament. Its medial surface forms part of the lateral wall of the nasal cavity; at its upper part is a rough, uneven area, which articulates with the ethmoid, closing in the anterior ethmoidal cells; below this is an oblique ridge, the ethmoidal crest, the posterior end of which articulates with the middle nasal concha, while the anterior part is termed the agger nasi; the crest forms the upper limit of the atrium of the middle meatus. The upper border articulates with the frontal bone and the anterior with the nasal; the posterior border is thick, and hollowed into a groove, which is continuous below with the lacrimal groove on the nasal surface of the body: by the articulation of the medial margin of the groove with the anterior border of the lacrimal a corresponding groove on the lacrimal is brought into continuity, and together they form the lacrimal fossa for the lodgement of the lacrimal sac. The lateral margin of the groove is named the anterior lacrimal crest, and is continuous below with the orbital margin; at its junction with the orbital surface is a small tubercle, the lacrimal tubercle, which serves as a guide to the position of the lacrimal sac. The Alveolar Process (processus alveolaris).—The alveolar process is the thickest and most spongy part of the bone. It is broader behind than in front, and excavated into deep cavities for the reception 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 subdivided into minor cavities by septa; those for the incisors are single, but deep and narrow. The Buccinator arises from the outer surface of this process, as far forward as the first molar tooth. When the maxillæ are articulated with each other, their alveolar processes together form the alveolar arch; the center of the anterior margin of this arch is named the alveolar point. The Palatine Process (processus palatinus; palatal process).—The palatine process, thick and strong, is horizontal and projects medialward from 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. 160) is concave, rough and uneven, and forms, with the palatine process of the opposite bone, the anterior three-fourths of the hard plate. It is perforated by numerous foramina for the passage of the nutrient vessels; is channelled at the back part of its lateral border by a groove, sometimes a canal, for the transmission of the descending palatine vessels and the anterior palatine nerve from the spheno-palatine ganglion; and presents little depressions for the lodgement of the palatine glands. When the two maxillæ are articulated, a funnel-shaped opening, the incisive foramen, is seen in the middle line, immediately behind the incisor teeth. In this opening the orifices of two lateral canals are visible; they are named the incisive canals or foramina of Stenson; through each of them passes the terminal branch of the descending palatine artery and the nasopalatine nerve. Occasionally two additional canals are present in the middle line; they are termed the foramina of Scarpa, and when present transmit the nasopalatine nerves, the left passing through the anterior, and the right through the posterior canal. On the under surface of the palatine process, a delicate linear suture, well seen in young skulls, may sometimes be noticed extending lateralward and forward on either side from the incisive foramen to the interval between the lateral incisor and the canine tooth. The small part in front of this suture constitutes the premaxilla (os incisivum), 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. It presents, close to its medial margin, the upper orifice of the incisive canal. The lateral border of the process is incorporated with the rest of the bone. The medial border is thicker in front than behind, and is raised above into a ridge, 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 named the incisor crest; it is prolonged forward into a sharp process, which forms, together with a similar process of the opposite bone, the anterior nasal spine. The posterior border is serrated for articulation with the horizontal part of the palatine bone. 
FIG. 160 - The bony palate and alveolar arch. Ossification.—The maxilla is ossified in membrane. Mall (*40 and Fawcett (*41 maintain that it is ossified from two centers only, one for the maxilla proper and one for the premaxilla. These centers appear during the sixth week of fetal life and unite in the beginning of the third month, but the suture between the two portions persists on the palate until nearly middle life. Mall states that the frontal process is developed from both centers. The maxillary sinus appears as a shallow groove 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 maxilla was formerly described as ossifying from six centers, viz., one, the orbitonasal, forms that portion of the body of the bone which lies medial to the infraorbital canal, including the medial part of the floor of the orbit and the lateral wall of the nasal cavity; a second, the zygomatic, gives origin to the portion which lies lateral to the infraorbital canal, including the zygomatic process; from a third, the palatine, is developed the palatine process posterior to the incisive canal together with the adjoining part of the nasal wall; a fourth, the premaxillary, forms the incisive bone which carries the incisor teeth and corresponds to the premaxilla of the lower vertebrates; (*42 a fifth, the nasal, gives rise to the frontal process and the portion above the canine tooth; and a sixth, the infravomerine, lies between the palatine and premaxillary centers and beneath the vomer; this center, together with the corresponding center of the opposite bone, separates the incisive canals from each other. 
FIG. 161 - Anterior surface of maxilla at birth. 
FIG. 162 - Inferior surface of maxilla at birth. Articulations.—The maxilla articulates with nine bones: two of the cranium, the frontal and ethmoid, and seven of the face, viz., the nasal, zygomatic, lacrimal, inferior nasal concha, palatine, vomer, and its fellow of the opposite side. Sometimes it articulates with the orbital surface, and sometimes with the lateral pterygoid plate of the sphenoid. Changes Produced in the Maxilla by AgeAt birth the transverse and antero-posterior diameters of the bone are each greater than the vertical. The frontal process is well-marked and the body of the bone consists of little more than the alveolar process, the teeth sockets reaching almost to the floor of the orbit. The maxillary sinus presents the appearance of 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.
Note 38. The number of teeth whose roots are in relation with the floor of the antrum 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 dens sapientiæ.” (Salter.) [back] Note 39. Aldren Turner (op. cit.) gives the following measurements as those of an average sized sinus: vertical height opposite first molar tooth, 1 1/2 inch; transverse breadth, 1 inch; and antero-posterior depth, 1 1/4 inch. [back] Note 40. American Journal of Anatomy, 1906, vol. v. [back] Note 41. Journal of Anatomy and Physiology, 1911, vol. xlv. [back] Note 42. Some anatomists believe that the premaxillary bone is ossified by two centers (see page 299). [back]
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