73 



OSTEOLOGY. 



circumference, and is generally deflected a little to one side. Its anterior border 

 articulates with the frontal spine and crest of the nasal bones. Its posterior, 

 divided into two parts, is connected by its upper half with the rostrum of the 

 sphenoid ; by its lower half with the vomer. The inferior border serves for the 

 attachment of the triangular cartilage of the nose. On each side of the perpen- 

 dicular plate numerous grooves and canals are seen, leading from foramina on 

 the cribriform plate ; they lodge filaments of the olfactory nerves. 



The Lateral Masses of the ethmoid consist of a number of thin-walled cellular 

 cavities, the ethmoidal cells, interposed between two vertical plates of bone, the 

 outer one of which forms part of the orbit, and the inner one part of the nasal 

 fossa of the corresponding side. In the disarticulated bone, many of these cells 

 appear to be broken ; but when the bones are articulated, they are closed in in 

 every part. The upper surface of each lateral mass presents a number of apparently 

 half- broken cellular spaces ; these, however, are completely closed in when articu- 

 lated with the edges of the ethrnoidal fissure 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 foramina, and open 

 on the inner wall of the orbit. The posterior surface also presents large irregular 

 cellular cavities, which are closed in by articulation with the sphenoidal turbi- 

 nated bones, and orbital process of the palate. The cells at the anterior surface 

 are completed by the lachrymal bone and nasal process of the superior maxillary, 

 and those below also by the superior maxillary. The outer surface of each lateral 

 mass is formed of a thin smooth square plate. of bone, called the os planum; it 

 forms part of the inner wall of the orbit, and articulates above with the orbital 

 plate of the frontal ; below, with the superior maxillary and orbital process of the 

 palate ; in front, with the lachrymal ; and behind, with the sphenoid. 



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

 there projects downwards and backwards 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 arti- 



Fig. 40. Ethmoid Bone. Inner Surface of Right culates with the ethmoidal process of 



the inferior turbinated bone. 



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 convo- 

 luted margin, the middle turbinated 

 bone. The whole of this surface is 

 rough, and marked above by numerous 

 grooves which run nearly vertically 

 downwards 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 another 

 thin convoluted plate of bone the middle turbinated bone. It extends along the 

 whole length of the inner surface of each lateral mass ; its lower margin is free 

 and thick, and its concavity, directed outwards, assists in forming the middle 

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

 middle meatus, that the anterior ethmoid cells, and through them the frontal sinuses, 

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

 The cellular cavities of each lateral mass, thus walled in by the os planum on 

 the outer side, and by its articulation with the other bones already mentioned, are 



