140 



OSTEOLOGY. 



Middle meatus 



FIG. 147. THE ETHMOID SEEK FROM BEHIND. 



regarded as comparable to the enlarged down-turned extremities of the horizontal 

 limb of the letter. 



The study of this bone will be much facilitated by cutting through the cribriform plate on 

 one side of the perpendicular plate, thus removing the ethmoidal labyrinth of one side and 



exposing more fully the central perpen- 

 Alar process Crista galli didllar lamina. 



The perpendicular plate, of ir- 

 regular pentagonal shape, forms the 

 superior part of the nasal septum. 

 Its superior border projects above the 

 level of the cribriform plate so as to 

 form a crest, which is much elevated 

 anteriorly, where it terminates in a 

 thick, vertical, triangular process, 

 called the crista galli, the interior of 

 which is filled with fine spongy 

 bone, but is occasionally pneumatic. 

 The superior edge of this process 

 is sharp and pointed, and affords 

 attachment to the falx cerebri. 

 In front of this process there is 

 a groove which separates the pro- 

 cessus alares (alar processes) which 

 project from the crista galijon either side. By articulation with the frontal bone 

 this groove is converted into a canal, the foramen caecum ; this, however, is not 

 always blind, but frequently transmits a vein to the roof of the nose. The 

 posterior border of the perpendicular plate is thin, and articulates with the crest of 

 the sphenoid. The posterior inferior border in the adult is ankylosed with the 

 vomer ; and the anterior inferior border, which is usually thicker than the others, 

 unites with the carti- 

 laginous nasal septum. 

 The anterior superior 

 border articulates with 

 the spine of the frontal 

 bone and with the 

 median crest formed by 

 the union of the two 

 nasal bones. The per- ( rbit 

 pendicular plate, which 

 is usually deflected to 

 one or other side, has 

 generally smooth sur- 

 faces, except above, where 

 they are channelled by 

 short and shallow 

 grooves leading to the 



foramina which pierce the cribriform plate; these are for the lodgment of the 

 olfactory nerves. 



The ethmoidal labyrinth is composed of exceedingly thin bone, enclosing 

 a large number of air-cells ; these are arranged in three groups an anterior, a 

 middle, and a posterior, the walls of which have been broken in front, above, 

 behind, and below, in the process of disarticulation. Laterally they are closed 

 in by a thin oblong lamina, the lamina papyracea or orbital plate, which forms 

 a part of the medial wall of the orbit, and articulates above with the orbital 

 part of the frontal, which here roofs in the ethmoidal cells. (The line of this 

 suture is pierced by two canals, the anterior and posterior ethmoidal foramina, 

 both of which transmit small ethmoidal vessels and nerves. In front, the lamina 

 papyracea articulates with the lacrimal bone ; whilst below, by its union with 



Anterior and posterior 

 ethmoidal grooves 



Alar process 



Infundibulum 



Middle meatus- 



Middle concha of the nose 



Perpendicular plate 



Uncinate process 

 FIG. 148. THE ETHMOID SEEN FROM THE RIGHT SIDE. 



