186 



THE SKELETON. 



the upper spongy bone. The foramina of the middle row are the smallest ; they 

 perforate the bone and transmit nerves to the roof of the nose. At the front part 

 of the cribriform plate, on each side of the crista galli, is a small fissure, which 

 transmits the nasal branch of the ophthalmic nerve ; and at its posterior part a 

 a triangular notch, which receives the ethmoidal spine of the sphenoid. 



The Perpendicular Plate (Fig. 148) is a thin, flattened lamella of bone, which 

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

 the septum of the nose. It is much thinner in the middle than at the circum- 

 ference, and is generally deflected a little to one side. Its anterior border articu- 

 lates with the nasal spine of the frontal bone and crest of the nasal bones. Its 

 posterior border, divided into two parts, articulates by its upper half with the eth- 

 moidal crest 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 perpendicular 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 ethm,oidal 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 at every part. 

 The upper surface of each lateral mass presents a number of apparently half- 

 broken cellular spaces ; these are closed in when articulated by the edges of the 

 ethmoidal 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 foramina, 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 

 tuibinated 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, 

 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 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- 



FIG. 148. Perpendicular plate of ethmoid (enlarged), shown by 

 removing the right lateral mass. 



