THE ETHMOID BONE. 



79 



FIG. 42. Perpendicular plate of ethmoid (enlarged), shown by 

 removing the right lateral mass. 



lular spaces ; these are closed in when articulated by the edges of the eth- 

 moidal notch of the frontal bone. Crossing this surface are two grooves on 

 each side, 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 also presents large, irregular cellular cavities, which 



are closed in by articula- 

 tion with the sphenoidal 

 turbinated bones and orbi- 

 tal process of the palate. 

 The cells at the anterior 

 surface are completed by 

 the lachrymal bone and 

 nasal process of the supe- 

 rior maxillary, and those 

 below also by the superior 

 maxillary. The outer sur- 

 face of each lateral mass 

 is formed of a thin, smooth, 

 oblong plate of bone, called 

 the os planum; it forms 

 part of the inner Avail of 

 the orbit, and articulates, 

 above, with the orbital 

 plate of the frontal ; below, 

 with the superior maxil- 

 lary ; in front, with the lachrymal ; and behind, with the sphenoid and orbital 

 process of the palate. 



From the inferior part of each lateral mass, immediately beneath the os planum, 

 there projects downward and backward an irregular lamina of bone, called the 

 unciform process, from its hook-like form : it serves to close in the upper part of 

 the orifice of the antrum, and articulates with the ethmoidal process of the inferior 

 turbinated bone. It is often broken in disarticulating the bones. 



The inner surface of each lateral mass forms part of the outer wall of the nasal 

 fossa of the corresponding side. It is formed of a thin lamella of bone, which 

 descends from the under surface of the cribriform plate, and terminates below in 

 a free, convoluted margin, the middle turbinated bone. The whole of this sur- 

 face is rough and marked above by numerous grooves, which run nearly verti- 

 cally downward from the cribriform 

 plate ; they lodge branches of the 

 olfactory nerve, which are distributed 

 on the mucous membrane covering the 

 bone. The back part of this surface 

 is subdivided by a narrow oblique 

 fissure, the superior meatus of the 

 nose, bounded above by a thin, curved 

 plate of bone, the superior turbinated 

 bone. By means of an orifice at the 

 upper part of this fissure the posterior 

 ethmoidal cells open into the nose. 

 Below, and in front of the superior 

 meatus, is seen the convex surface of 

 the middle turbinated bone. It extends 

 along the whole length of the inner surface of each lateral mass ; its lower mar- 

 gin is free and thick, and its concavity, directed outward, assists in forming the 

 middle meatus. It is by a large orifice at the upper and front part of the middle 

 meatus that the anterior ethmoidal cells, and through them the frontal sinuses, 

 communicate with the nose by means of a funnel-shaped canal, the infnndibulum. 



FIG. 43. Ethmoid bone. Inner surface of right 

 lateral mass (enlarged). 



