ETHMOID BONE. 155 



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 triangular notch, which receives 

 the ethmoidal spine of the sphenoid. 



The Perpendicular Plats (Fig. 106) 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 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 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 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 at 

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

 rently 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 articulation with the sphenoidal turbinated 

 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 formi it serves to close in the 

 upper part of the orifice of the antrum, and articulates with the ethmoidal pro- 

 cess of the inferior turbinated bone. It is often broken in articulating the bones. 



The inner surface of each internal 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 m S- 107. Ethmoid Bone. Inner Surface of 

 belowin afree convoluted margin, the Ri S ht Lateral Mass (enlarged), 



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 su- 

 perior 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 



