154 



OSTEOLOGY 



plate, on either side of the crista galli, is a small fissure which is occupied by a 

 process of dura mater. Lateral to this fissure is a notch or foramen which trans- 

 mits the nasociliary nerve; from this notch a groove extends backward to the 

 anterior ethmoidal foramen. 



FIG. 150. Perpendicular plate of ethmoid. Shown by removing the right labyrinth. 



Perpendicular Plate (lamina perpendicularis; vertical plate}. The perpendicular 

 plate (Figs. 150, 151) is a thin, flattened lamina, polygonal in form, which descends 

 from the under surface of the cribriform plate, and assists in forming the septum 

 of the nose; it is generally deflected a little to one or other side. The anterior border 

 articulates with the spine of the frontal bone and the crest of the nasal bones. 

 The posterior border articulates by its upper half with the sphenoidal crest, by its 

 lower with the vomer. The inferior border is thicker than the posterior, and serves 

 for the attachment of the septal cartilage of the nose. The surfaces of the plate 

 are smooth, except above, where numerous grooves and canals are seen; these 

 lead from the medial foramina on the cribriform plate and lodge filaments of the 

 olfactory nerves. 



The Labyrinth or Lateral Mass (Idbyrinthus ethmoidalis) consists of a number 

 of thin-walled cellular cavities, the ethmoidal cells, arranged in three groups, 



anterior, middle, and posterior, and inter- 

 posed between two vertical plates of bone; 

 the lateral plate forms part of the orbit, 

 the medial, part of the corresponding 

 nasal cavity. In the disarticulated bone 

 many of these cells are opened into, but 

 when the bones are articulated, they are 

 closed in at every part, except where 

 they open into the nasal cavity. 



Surfaces. The upper surface of the laby- 

 rinth (Fig. 149) presents a number of 

 half-broken cells, the walls of which are 

 completed, in the articulated skull, by 

 the edges of the ethmoidal notch of the 

 frontal bone. Crossing this surface are 

 two grooves, converted into canals by articulation with the frontal; they are the 

 anterior and posterior ethmoidal canals, and open on the inner wall of the orbit. 

 The posterior surface presents large irregular cellular cavities, which are closed in 



Crista galli 



Labyrinth 



Superior nasal 



concha 

 Superior meatus 



Uncinate process 



Middle nasal concha 

 Perpendicular plate 

 Fio. 151. Ethmoid bone from behind. 



