20 ANATOMY OF THE RAT 



internal ridge marks the boundary between the cerebral 

 and olfactory fossae. The cribriform plate is attached to 

 the frontal bones between the olfactory fossa and nasal 

 cavity. 



Ethmoid Bone (Os Ethmoidals) 



This highly complicated bone, of cartilaginous origin, 

 is the chief representative of the embrycmic nasal cap- 

 sule. It includes the perpendicular plate of the nasal 

 septum, the cribriform plate, and the ethmoturbinal bones. 



Maxillary Bone (Maxilla) 



The maxillary bone consists chiefly of five processes — 

 the frontal, zygomatic, alveolar, palatine, and orhitai. It 

 is a membrane bone. The frontal process is divided by 

 the large infraorbital foramen into a lateral and median 

 portion. The latter forms part of the lateral wall of the 

 nasal cavity and is comparatively thin. The skull is un- 

 ossified posterior to this part of the frontal process, 

 which joins the premaxillary bone dorsally and anteriorly. 

 The lateral portion of the frontal process is a laterally 

 concave plate connected with the dorsal surface of the 

 cranium by a bony arch. To the dorsal end of this plate 

 is attached the zygomatic process, which forms the an- 

 terior half of the zygomatic arch. The alveolar process 

 (so called because it contains the alveoli of the teeth) is 

 a strong bony mass holding the three molar teeth. It 

 is bounded posteriorly and dorsomedially by the sphenoid 

 bones, medially by the palatine process and palatine bone. 

 The palatine process joins its fellow on the opposite side 

 by a median suture in the roof of the mouth. It forms 

 part of the hard palate. In front it surrounds the pos- 

 terior half of the prepalatine foramen. It joins the pala- 



