40 ETHMOID BONE. 



Fig. 22.* branch of the ophthalmic nerve. 



In the groove of this lamella the 

 foramina pierce the bone complete- 

 ly, but at either side they are the 

 apertures of canals, which run for 

 some distance in the substance of 

 the central lamella, inner wall of 

 the lateral mass and spongy bones. 

 The opening for the nasal nerve is 

 a narrow slit in the anterior part of 

 of the cribriform lamella, close to 

 the crista galli. The cribriform 

 lamella serves to connect the la- 

 teral masses with the perpendicu- 



Iar P late - 



The lateral masses (labyrinth!) 

 are divisible into an internal and 



external surface, and four borders, superior, inferior, anterior, and 

 posterior. The internal surface is rough and slightly convex, and 

 forms the external boundary of the upper part of the nasal fossae. 

 Towards the posterior border of this surface is a narrow horizontal 

 fissure, the superior meatus of the nose, the upper margin of which is 

 thin, and somewhat curled inwards ; hence it is named the superior 

 turbinated bone (concha superior). Below the meatus is the convex 

 surface of another thin plate, which is curled outwards, and forms the 

 lower border of the mass, the middle turbinated bone (concha media). 

 The external surface is quadrilateral and smooth, hence it is named 

 os planum, and, from its thinness, lamina papyracea ; it enters into the 

 formation of the inner wall of the orbit. 



The superior border is irregular and cellular, the cells being com- 

 pleted by the edges of the ethmoidal fissure of the frontal bone. This 

 border is crossed by two grooves, sometimes complete canals, opening 

 into the orbit by the anterior and posterior ethmoidal foramina. The 

 inferior border is formed internally by the lower border of the middle 

 turbinated bone, and externally by a concave irregular fossa, the upper 

 boundary of the middle meatus. The anterior border presents a 

 number of incomplete cells, which are closed by the superior maxillary 



* The ethmoid bone seen from above and behind. 1. The central lamella. 

 2, 2. The lateral masses ; the numbers are placed on the posterior border of 

 the lateral mass at each side. 3. The crista galli process. 4. The cribriform 

 plate of the left side, pierced by the cribriform foramina. 5. The hollow space 

 immediately above and to the left of this number is the superior meatus. 6. 

 The superior turbinated bone. 7- The middle turbinated bone ; the numbers 

 5, 6, 7, are situated upon the internal surface of the left lateral mass, near its 

 posterior part. The interval between these parts is the superior meatus. 8. 

 The external surface of the lateral mass, or os planum. 9. The superior or 

 frontal border of the lateral mass, grooved by the anterior and posterior eth- 

 moidal canals. 10. Refers to the concavity of the middle turbinated bone, 

 which is the upper boundary of the middle meatus. 



