OSSIFICATION OF THE BONES OF THE HEAD. 



77 



the fifth or sixth year. The ethmoidal cells appear in the sixth month as depressions of the 

 mucous membrane, but bony walls are not developed until after birth. 



The superior maxillary bone begins to ossify immediately after the clavicle and the 

 lower jaw. The osseous deposit takes place at several points, but the different parts speedily 

 fuse, and the precise number of centres is by no means certain. It is probable, however, that 

 the maxilla proper is developed in at least three pieces, a malar portion external to the infra- 

 orbital canal, an orbito-facial portion between the foregoing and the nasal fossa, and a 

 palatine portion including the palate process and the adjoining part of the nasal wall. The 

 part of the bone which carries the incisor teeth, extending as far back as the incisor foramen, 

 has an independent origin, corresponding to the premaoclllary bone of the lower animals. In 

 the young subject always, and often in the adult, there is to be seen a fine incisor fissure on 

 the under surface of the palate process, passing outwards from the anterior palatine fossa to 

 the alveolar border, internal to the canine socket ; and on the upper surface a similar line 

 may be seen, though less frequently, extending up some distance on the nasal surface of the 

 body ; but no trace of the line of union exists on the facial surface, as is the case in the 

 lower animals. This is due to the development at the lower and fore part of the maxilla of 

 an outgrowth, termed the incisor process, which forms the front wall of the incisor sockets ; 



Fig. 79. DIFFERENT VIEWS OF THE SUPERIOR MAXILLARY BONE OF A FCETUS OF FOUR OR FIVE 



MONTHS. (R. Quain.) 



A, external surface ; a fissure, 1, is seen extending from the orbital surface into the infraorbital 

 canal. 



B, internal surface ; the incisor fissure, 2, extends from the foramen upwards through the hori- 

 zontal plate and some way into the nasal process. 



C, the hone from below, showing the imperfect alveoli and the incisor fissure, 2', 1, which crosses 

 the palate plate to the septum between the second and third alveoli. 



behind this process, between it and the palate process, is a deep groove in which the pre- 

 maxillary bone is formed, the latter being distinct up to the fifth month of foetal life. 

 (Callender, Phil. Trans., 1869 ; Th. Kolliker," Os intermaxillare des Menschen," 1882 ; Sutton. 

 Proc. Zool. Soc., 1884.) 



The sockets of the teeth are formed by the downgrowth of an outer and an inner plate on 

 the sides of the dental groove, and subsequently the partitions appear, those on each side of 

 the canine tooth first. The antrum appears as a shallow depression on the inner surface of 



Fig. 80. THE VOMER AT THE TIME OF BIRTH. (R. Quain.) 



1 & 2, the two plates forming a groove for the reception of the septal 

 cartilage. 



the bone at about the fourth month ; this gradually extends, separating the orbital and 

 palate portions of the bone, which at birth are still very close together. At its first appear- 

 ance the antrum has a cartilaginous wall, which afterwards becomes absorbed. The infra- 

 orbital canal begins aa a groove on the orbital surface, which is gradually closed by the 

 growing over of the outer margin ; but a fine suture remains indicating the line of meeting. 



The palate bone is ossified from a single centre, which appears in the seventh or eighth 

 week at the angle between its horizontal and vertical parts. 



The vomer is ossified from a pair of nuclei which arise in membrane at the lower and 

 back part of the cartilaginous nasal septum in the eighth week. They soon unite below, but 

 growing forwards and upwards form the two laminae which embrace the septal cartilage. 

 These laminae gradually undergo increased union from behind forwards till the age of 

 puberty, thus forming a median plate, with only a groove remaining on its anterior and 

 superior margins. 



The nasal and lachrymal bones are each ossified from a single centre appearing about the 

 eighth, week. At the time of birth there is a layer of cartilage beneath the nasal bone, which 



