ETHMOID BOXE. 



45 



ETHMOID BONH 



The ethmoid, or sieve- like bone, projects downwards from between 

 the orbital plates of the frontal bone, and enters into the formation of 

 the cranium, the orbits, and the nasal fossae. It is of a cuboid fiij^ure. 

 It is exceedingly light for its size, being composed of verj^ thin plates 

 of bone forming in part irregular cells. It consists of a crniral vertical 

 plate, and of two lateral masses, the ethino-lurhinals, united at their 

 superior extremities by the horizontal cribriform lamella. 



It articulates with tliirteen bones ; the frontal, sphenoid and A-omer, 

 the nasal;, lachrymal, superior maxillary, palatal, and inferior turbinated 

 bones. 



The vertical plate lies in the mesial plane, and forms the upper 

 part of the septum of the nose. Its superior margin appears in the 

 cranial cavity, above the cribriform lamella, in the form of a ridge 



B 



Fig. 38.— The Ethmoid Bone. (A. T.) | 



A, from the right side. 1, ciista galli ; 2, 

 vertical plate ; 3, cribriform plate and foramina ; 

 4, orbital plate or os planum ; 5, 5, the uncinate 

 process ; 6, the superior, and 7, the inferior 

 turbinated bone ; 8, the anterior, and 9, the 

 posterior ethmoidal groove or foramen. 



B, the ethmoid bone from beliind. The in- 

 dications where marked are the same as in A. 

 10, the lateral or cellular pai't of the bone ; 11, 

 its posterior surface of union with the sphenoidal 

 turbinated and palate bones. 



which rises anteriorly into a thick 

 process, the crista galli, to which the 

 falx cerebri is attached. The an- 

 terior margin of the crista galli is 

 vertical and broad, usually present- 

 ing a groove, which completes the 

 foramen ctccum of the frontal bone. 

 Below the level of the cribriform 

 lamella, the anterior margin of the 

 vertical plate articulates with the 



nasal spine of the frontal and occasionally with the nasal bones. The 

 inferior margin articulates in front, and sometimes even in its whole 

 extent, with the septal cartilage of the nose ; and in its posterior half, in 

 the adult, it is more or less completely joined by osseous union on one 

 or both sides to the two plates or alie of the vomer. The posterior 

 margin is very thin, and is united to the crest of the sphenoid. 

 This plate presents superiorly a number of grooves and minute 

 canals leading from the foramina of the cribriform lamella, for the 

 transmission of the olfactory nerves. 



The lateral masses enclose a number of spaces of irregular form, 

 arranged in two sets, the anterior and posterior ethmoidal cells, 

 whicli in the recent state are lined with prolongations of the mucous 

 membrane of the nose. On the external aspect of each lateral mass 

 is a thin, smooth lamina, of a quadrilateral form, the orbital plate or 

 ■OS planum, which closes in the etlnnoidal cells, and forms a consider- 



