92 Anatomy of the Rabbit. 



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 W'hich the roof of the nasal and cranial cavities has been re- 

 moved. 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 por- 

 tions 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, pos- 

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

 cavity. It is united with the cartilaginous nasal septum and also with 

 the presphenoid. It forms the terminal member of the chain of bones 

 lying in the basicranial axis. 



The ethmoidal labyrinth (labyrinthus ethmoidalis) occupies for the 

 most part the posterior portion of the nasal fossa, but the nasoturbinal 

 extends forward to its anterior end, and is attached for the greater part 

 of its length to the internal surface of the nasal bone. It is broadest in 

 its middle portion, where it projects into the space left between the 

 ethmoturbinal proper and the maxilloturbinal, and contains at this point 

 a pouch-like cavity, termed the marsupium nasale. The whole struc- 

 ture is comparable to one of the folds of the ethmoturbinal proper; 

 but it is frequently seen to be divided into anterior and posterior parts 

 by a thin vertical line of cartilage, the anterior division being probably 

 allied to the maxilloturbinal. Its middle, ventral, portion bears a 



