THE BONES OF THE HEAD 103 



lated skull its size is considerably diminished by the perpendicular 

 plate of the palate bone behind, the maxillary process of the in- 

 ferior turbinate below, and above this by the uncinate process of 

 the ethmoid. The opening is further curtailed by the adjacent 

 mucous membrane. Under these circumstances it is reduced to 

 a small aperture, situated near the upper part, which opens into the 

 middle meatus of the nasal fossa. The antrum is usually uni- 

 locular, but it may be partially divided into compartments. 



The bone derives its blood - supply from many sources, such 

 as posterior dental, infra-orbital, middle and anterior dental, 

 facial, frontal, superior or descending palatine, and naso-palatine 

 arteries. 



Articulations. — ^The superior maxilla articulates with nine bones, 

 as follows: malar, nasal, frontal, lachrymal, ethmoid, inferior 

 turbinate, palate, vomer, and its fellow of the opposite side. In 

 addition to these it may articulate with the pterygoid process and 

 great wing of the sphenoid. 



Fig. 62. — The Fcetal Superior Maxillary Bone. 



A, External View, showing the Infra-orbital Groove and Foramen, with 

 Fissure ; B, Inferior View, showing the Incisor Fissure and Alveoli ; 

 C, Internal View, showing the Antrum and Incisor Fissure. 



Structure. — The bone contains the maxillary air sinus. For the 

 most part cancellated tissue is absent, except in the alveolar process 

 around the alveoli. 



Ossification. — ^The superior maxillary bone is ossified in membrane. Accord- 

 ing to ^lalland Fawcett it has two centres — maxillary proper and premaxillary, 

 which appear about the sixth week, and join about the third month. The 

 centre referred to as maxillary proper appears in the region of the future 

 canine alveolus, and from it ossification proceeds backwards into the malar 

 process, upwards into the posterior half of the nasal process, inwards into 

 the posterior three-quarters of the palatal process, and downwards into the 

 alveolar border, excluding the incisor portion. 



The premaxillary centre gives rise to the premaxilla, which Ues on the mesial 

 side of the maxilla proper and bears the upper incisor teeth. It also gives 

 rise to (i) the anterior fourth of the palatal plate, and (2), according to Fawcett, 

 the anterior half of the nasal process. It is to be noted that, whilst the 

 anterior half of the nasal process is ossified from the premaxillary centre, the 

 posterior halfoi that process (bearingthelachyrmal groove) is ossified from the 

 maxillary proper centre. 



Besides the premaxillary centre there is an infravomerine centre, according 

 to Rambaud and Renault. This centre Ues beneath the anterior part of the 

 vomer, and it gives rise to the infravomerine part of the bone, which forms 

 the mesial wall of Stensen's canal. The Une of union between the premaxilla 



