ETHMOID BONE. 73 



bones. The opening for the nasal nerve is a nar- Fig. 33 * 



row slit in the anterior part of the cribriform la- 

 mella, close to the crista galli. The cribriform 

 lamella serves to connect the lateral masses with 

 the perpendicular plate. 



The lateral masses (labyrinthi) 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, w r hich 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 completed 

 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 bor- 

 der 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 and lachrymal bone ; and 

 the posterior border is regularly cellular, to articulate with the sphenoid 

 and palate bones. 



The lateral masses are composed of cells, which are divided by a thin 

 partition into anterior and posterior ethmoidal cells. The anterior, the 

 most numerous, communicate with the frontal sinuses, and open by means 

 of an irregular and incomplete tubular canal, the infundibulum, into the 

 middle meatus. The posterior calls, fewer in number, open into the su- 

 perior meatus. 



Development. By three centres : one for each lateral mass, and one for 

 the perpendicular lamella. Ossification commences in the lateral masses 

 at about the beginning of the fifth month, appearing first in the os planum 

 and then in the spongy bones. During the latter half of the first year 

 after birth, the central lamella and lamina cribrosa begin to ossify, and are 

 united to the lateral masses by the beginning of the second year. The 

 cells of the ethmoid are developed in the course of the fourth and fifth year. 



* 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 ma?s at 

 each side. 3. The crista galli process. 4. The cribriform plate of the left side, pierced 

 by its foramina. 5. The hollow space immediately above and to the left of this num- 

 ber 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 ethmoidal canals. lO. Re/ers 

 to the concavity of the middle turbinated bone, which is the upper boundary of die 

 middle meatus. 



7 



