THE BONES OF THE HEAD 97 



that process, downwards into the perpendicular plate, and outwards into 

 the cribriform plate, into which latter osseous matter also extends inwards 

 from each lateral mass. The three original parts unite about the fifth year. 

 The osseous ethmoidal cells usually make their appearance about the third year. 



The Superior Maxillary Bones. 



The superior maxillary bone forms, with its fellow, a large part 

 of the face, and, besides supporting the upper teeth of its own 

 side, it enters into the formation of the orbit, nasal fossa, and 

 hard palate. It is composed of a central portion or body, and 

 four processes — nasal, malar, ah^eolar, and palatal. 



The body is excavated into a large cavity, called the antrum of 

 Highmore or maxillary air sinus, and it presents four surfaces — 

 antero-external, postero-extemal, superior, and internal. 



The 'antero-external or facial surface is limited above by the 

 infra-orbital border, below by the alveolar border, mternally by the 

 mesial border, presenting the nasal notch, and externally by the 

 malar process and a ridge of bone extending downwards from it 

 to the first molar alveolus. It presents inferiorly five ridges, 

 coinciding with the roots of the incisor, canine, and bicuspid 

 teeth, of which that of the canine is conspicuous, and is called the 

 canine ridge. Internal to this ridge is the incisor or myrtilorm 

 fossa (like a myrtle-berry), which gives origin internally to the 

 depressor alae nasi, and externally to a deep slip of the orbicularis 

 oris, whilst above, and external to, the latter the compressor naris 

 arises. External to the canine ridge is the canine fossa, which, 

 at its upper part, gives origin to the levator anguli oris, the bone 

 being here thin and translucent in front of the antrum. Above 

 the canine fossa, near the infra-orbital border, is the infra-orbital 

 foramen, which is the outlet of the infra-orbital canal, and trans- 

 mits the infra-orbital nerve and vessels. Immediately above this 

 foramen the levatc ; labii superioris arises. The mesial border 

 of the facial surface presents the deep nasal notch, at the lower 

 and inner part of which is a sharp projection, forming, with its 

 fellow, the anterior nasal spine, below which the border is vertical. 



The postero-external or zygomatic surface is situated behind the 

 malar process and the ridge connecting that process with the first 

 molar alveolus. Superiorly it is hmited by the posterior border 

 of the orbital surface, inferiorly by the molar portion of the alveolar 

 border, and posteriorly by the posterior border of the bone. It 

 looks into the zygomatic and spheno-maxillary fossae, and its 

 outhne is convex. Towards the centre it presents the openings 

 of two or three posterior dental canals, which lead to the molar 

 alveoli, and transmit branches of the posterior superior dental 

 nerve and artery. At the lower and back part this surface gives 

 rise to the tuberosity, which lies above and behind the last molar 

 tooth. This tuberosity articulates with the pyramidal process of 

 the palate bone, and gives origin to some fibres of the internal 

 pterygoid muscle. 



7 



