

THE LACRIMAL BONE 



163 



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 forw r ard into a sharp process, which forms, together with a similar 

 process of the opposite bone, the anterior nasal spine. The posterior border is ser- 

 rated for articulation \vith the horizontal part of the palatine bone. 



Ossification. The maxilla is ossified in membrane. Mall 1 and Fawcett 2 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 



Fia. 161. Anterior surface of maxilla at birth. 



FIG. 162. Inferior surface of maxilla at birth. 





leeth and corresponds to the premaxilla of the lower vertebrates; 3 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 

 ^he corresponding center of the opposite bone, separates the incisive canals from each other. 



Articulations. The maxilla articulates with nine bones: two of the cranium, the frontal and 

 :thmoid, and seven of the face, viz., the nasal, zygomatic, lacrimal, inferior nasal concha, palatine, 

 /omer, and its fellow of the opposite side. Sometimes it articulates with the orbital surface, 

 md sometimes with the lateral pterygoid plate of the sphenoid. 



CHANGES PRODUCED IN THE MAXILLA BY AGE. 



At 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. 



The Lacrimal Bone (Os Lacrimale). 



The lacrimal bone, the smallest and most fragile bone of the face, is situated 

 at the front part of the medial wall of the orbit (Fig. 164). It has two surfaces 

 and four borders. 



1 American Journal of Anatomy, 1906, vol. v. 



2 Journal of Anatomy and Physiology, 1911, vol. xlv. 



3 Some anatomists believe that the premaxillary bone is ossified by two centers (see page 299). 



