THE ETHMOID BONE. 49 



of one or two irregular projections, with the ethmoidal process of the inferior tur- 

 binate bone (fig. 69, p. 66). 



The internal aspect of each lateral mass forms part of the external wall of the 

 nasal fossa, and consists of a thin, uneven lamella, connected above with the cribri- 

 form plate, and exhibiting a number of canals and grooves for branches of the 

 olfactory nerve. It is divided at its back part by a channel, directed forwards from 

 its posterior margin to about its middle. This is the superior meatus of the nose, 

 and communicates with the posterior ethmoidal cells. The short, thin plate which 

 overhangs this channel, is the superior tur- 

 binate process or spongy bone. Below the 

 groove is another plate, somewhat thick- 

 ened and rolled outwards inferiorly, the 

 inferior turbinate process or middle spongy 

 bone. This is free also in front and 



ANT. ETHM. CELLS 



NFUNDIBULUM 



ORM PLATE 



SUP.TUR3. BONE 

 'P. MEATUS 



0. TURS. BONE 



Fig. 48. RIGHT LATERAL MASS OF ETHMOID BONE 

 INNER SURFACE. (Drawn by D. Gunn.) 



Fig. 49. THE ETHMOID BONE, FROM ABOVE. 

 (Drawn by D. Gunn.) 



behind, and, as has already been seen on the outer aspect, overhangs the middle 

 meatus of the nose. 



The superior margin of the lateral mass is covered, and the cells completed, by 

 the projecting inner border of the orbital plate of the frontal bone ; two grooves are 

 seen crossing it, which complete with the frontal bone the internal orbital canals. 

 The inferior margin is formed by the rounded edge of the middle turbinate bone, 

 and is free in the nasal fossa. The anterior extremity presents one or two open 

 cells, which are closed by the nasal process of the superior maxilla ; and the posterior 

 extremity fits against the front of the body of the sphenoid, where it is commonly 

 anchylosed with the sphenoidal spongy bone. 



The cribriform plate corresponds in size to the ethmoidal notch of the frontal 

 bone, which it occupies. On each side of the crista galli it is depressed into the 

 olfactory groove which lodges the olfactory bulb, and is pierced by numerous 

 foramina for transmission of the filaments of the olfactory nerves. The foramina 

 in the middle of the groove are few, and simple perforations ; the internal and 

 external sets, more numerous, are the orifices of small canals which subdivide as 

 they descend on the vertical plate and lateral mass. At the anterior extremity is a 

 small fissure on each side of the crista galli, close to its base ; and externally to this 

 is a notch or foramen, connected by a groove with the anterior internal orbital 

 canal, which transmits the nasal branch of the ophthalmic nerve. 



VOL. II. 



