44 ETHMOID BONE. 



ETHMOID BONE. The ethmoid (^M-OJ, a sieve) is a square-shaped 

 cellular bone, situated between the two orbits, at the root of the nose, 

 and perforated upon its upper surface by a number of small open- 

 ings, from which peculiarity it has received its name. It consists of 

 a perpendicular lamella and two lateral masses. 



The perpendicular lamella is a thin central plate, which arti- 

 culates with the vomer and cartilage of 

 the septum, and assists in forming the sep- 

 tum of the nose. It is surmounted supe- 

 riorly by a thick and strong process, the 

 crista galli, which projects into the cavity 

 of the skull, and gives attachment to the 

 falx cerebri. On each side of the crista 

 galli, upon the upper surface of the bone, 

 is a thin and grooved plate, perforated by 

 a number of small openings, the cribriform 

 lamella, which supports the bulb of the 

 olfactory nerve, and gives passage to its 

 filaments, and to the internal nasal nerve. 

 The cribriform lamella serves to connect masses with the perpen- 

 dicular plate. 



The lateral masses are divisible into an internal and external sur- 

 face, 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 tur- 

 binated bone. 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. The external surface is quad- 

 rilateral and smooth, hence it is named os planum ; 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, open- 

 ing 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 



* 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 g-alli process. 4. The cribrifrom 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 sur- 

 face 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 poste- 

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

 which is the upper boundary of the middle meatus. 



