I50 Ax ATOMY OF THE R.ABBIT. 



premaxillary bones. Two processes are thus formed, one medial, 

 the other lateral to the nasal. The medial process is associated 

 with that of the opposite side to form a triangular frontal spine, 

 while the lateral or maxillary process (processus maxillaris) 

 projects forward between the nasal and premaxillary bones, on 

 the one hand and the subcutaneous process of the lacrimal, the 

 orbital process of the maxilla, and the body of the latter, on the 

 other. 



The orbital portion of the frontal forms a considerable portion 

 of the orbital wall. Its anterior margin is in contact with the 

 lacrimal bone, its ventral margin with the slender sphenoorbital 

 process of the maxilla, the ethmoid process of the orbitosphenoid, 

 and the orbitosphenoid proper. Its internal surface is divided by a 

 vertical ridge into anterior and posterior portions, in relation 

 respectively to the anterior and middle cranial fossae. The anterior 

 cranial fossa is enclosed by the frontal bones, with the exception, 

 however, of a small portion of the floor which is formed by the 

 cribriform plate of the ethmoid. 



10. THE ETHMOID BONE. 



The ethmoid bone (os ethmoidale), the chief representative of 

 the embryonic cartilaginous nasal capsule, is a delicate, greatly 

 sculptured structure, almost completely enclosed by the membrane 

 bones of the face. Its features may be studied either in the divided 

 skull, or in one from which the roof of the nasal and cranial cavities 

 has been removed. It consists of three main portions, namely, the 

 cribriform plate, the perpendicular plate, and the paired 

 lateral masses, or ethmoidal labyrinths. 



The cribriform plate (lamina cribrosa) is exposed to the 

 anterior cranial fossa. It is somewhat heart-shaped, with its apex 

 in contact with the ethmoidal processes of the orbitosphenoids. 

 Its lateral portions are perforated by numerous foramina, giving 

 passage in the natural condition to the branches of the olfactory 

 nerves. Its median portion forms a low vertical ridge, the crista 

 galli, continuous in front with the perpendicular plate. 



The perpendicular plate (lamina perpendicularis) is the bony, 

 posterior portion of the nasal septum, and as such is exposed to the 



