THE BONES OF THE SKULL 193 



The Premaxilla 



The premaxilla or incisive bone (os incisivum) forms the 

 anterior portion of the upper jaw. It comprises a central portion, 

 the body-^including with the latter the scarcely differentiated 

 alveolar portion containing the large and small incisors — a frontal 

 process, and a palatine process. The body forms a portion of the 

 palatal surface of the skull and of the lateral boundary of the 

 incisive foramen. Its dorsal surface forms part of the boundary of 

 the piriform aperture, the remaining portion of this being formed 

 b}^ the nasal bone. The palatine process extends backward on the 

 medial side of the bone, closely appHed on the palatal surface to 

 its fellow of the opposite side, and forms in this way a medial 

 boundary for the incisive foramen. Its dorsal surface, in conjunc- 

 tion with that of the corresponding process of the other side, bears 

 a broad palatine groove (sulcus palatlnus), lodging a portion of 

 the cartilage of the vomeronasal organ and nasopalatine duct. 

 The frontal process (processus frontalis) is a thin bony splint, 

 extending backward between the nasal and maxillary bones, and 

 terminating between the former and the maxillary process of the 

 frontal. 



The Zygomatic Bone 



The zygomatic bone (os zygomaticum) is a separate element 

 only in very young animals. In the adult it is fused anteriorly 

 with the zygomatic process of the maxilla, the position of the 

 original suture being roughly identifiable as the point where the 

 free horizontal portion of the zygomatic arch arises from the trans- 

 verse zygomatic process. It forms an almost sagittal plate of bone 

 bridging the orbit and serving for the attachment of the masseter 

 muscle of the mandible. Its dorsal margin forms posteriorly a 

 smooth, horizontal articulation with the zygomatic process of the 

 squamosal, the end of the bone projecting considerably behind the 

 articulation. 



The Nasal Bone 



The nasal bone (os nasale) Is a thin, elongated bone forming 

 the roof of the nasal fossa and, in conjunction with Its fellow of the 

 opposite side, the dorsal boundary of the piriform aperture. It is 



