20 



Bones of the Skull. 



Labyrinth us 



< Ilimoidalis 



Crista galli 



' Processus alaris 



Lamina cribrosa 



Lamina 

 papyracea 



Labyriiithns 

 etbmoidalis 



Lamina cribrosa 



Processus uncinatus 

 Concha nasalis superior 



Concha nasalis media 

 Lamina perpendicularis 



--Foramen ethmoidale 

 posterius 



Lamina papyracea 



Foramen ethmoidale 

 anterius 



Crista galli 

 Processus alaris 



Lamina perpendicularis 



21 and 22. Ethmoid bone, os ethmoidale. 



From behind, somewhat schematic. From above. 



Ths os ethmoidale (ethmoid bone} (see also Figs. 2325, 63, 64, 6870, 7375) 

 is unpaired, helps in part to close the most anterior portion of the envbral skull below, but 

 reaches with its main mass deep down into the facial skull and helps to form the nose and 

 orbit. It consists above of a transversely placed oblong plate, the lamina cribrosa; from the 

 under surface of this extending downward in the median plane is the lamina perpendicularis 

 and from each of its two lateral borders hangs the labyrinthus ethmoidalis. 



The lamina cribrosa (cribriform plate) (see also Figs. 23, 6366 and 72 75) lies 

 horizontally, fills the incisura ethmoidalis oss. frontalis completely and thus forms at the 

 anterior and the two lateral margins a part of the sulura frontoethmoidalis ; the posterior 

 margin lies on the anterior border of the upper surface of the body of the sphenoid bone 

 (sutura sphenoethmoidalis). The cribriform plate presents on its upper surface in the median 

 plane a ridge which becomes elevated in front to form the oblong crista fjalli or cock's comb 

 (for the falx cerebri) ; this is higher in front than behind and often contains bone-marrow 

 spaces; it is attached to the pars nasalis and crista frontalis oss. frontalis, usually by means 

 of the two small, laterally directed processus atares , and completes the entrance to the 

 foramen caecum oss. frontalis when this is incomplete. The cribriform plate is perforated by 

 numerous foramina of different sizes, the larger of w r hich are arranged often in two rows, one 

 medial and one lateral, and which are continued in small grooves on the lamina perpendicularis 

 and on the medial surface of the labyrinth; they give passage to the nn. olfactorii. A small 

 slit lateralward from the anterior part of the crista galli gives passage to the a. and the n. 

 ethmoidalis anterior. 



