AXIAL SKELETON 



313 



Sphenoid Bone. — Ten principal centers arise in the cartilage that corre- 

 sponds to this bone (Fig. 318) : (1 and 2) in each ala magna (alisphenoid) ; (3 and 

 4) in each ala parva (orbitosphenoid) ; (5 and 6) in the corpus between the alse 

 magnae {basis phenoid) ; (7 and 8) in each lingula; (9 and 10) in the corpus between 

 the alae parvae (presphenoid) . Intramembranous bone also enters into its compo- 



Ala magna Ala parva 



(Alisphenoid) Presphenoid (Orbitosphenoid) 



Lingula'^ j \'- Pterygoid 



Basisphenoid process 



Fig. 318. — Sphenoid bone of a human fetus 

 of nearly four months (after Sappey) . Parts still 

 cartilaginous are represented in stipple. 



Nasal septum 

 ..^Perpendicular plate 

 ..Crista galli 



■Cribriform plate 



-Labyrinth 



Fig. 319. — Ethmoid bone of a human fetus of 

 four months (modified after Kollmann). 



Squamosum 



sition, forming the orbital and temporal portion of each ala magna and the mesial 

 laminae (Fawcett) of each pterygoid process (except the hamulus). Fusion of the 

 various parts is completed during the first year. 



Ethmoid Bone. — The ethmoid cartilage consists of a mesial mass, which 

 extends from the sphenoid to the tip of the nasal process, and of paired masses 

 lateral to the olfactory fossae. The lower part 

 of the mesial mass persists as the cartilaginous 

 nasal septum, but ossification of the upper por- 

 tion produces the lamina perpendicularis and 

 the crista galli (Fig. 319). The lateral masses 

 ossify at first into the spongy bone of the eth- 

 moidal labyrinths. From this the definitive 

 honeycomb structure {ethmoidal cells) and the 

 conchce are formed through evaginations of the 

 nasal mucous membrane and the coincident re- 

 sorption of bone. (Similar invasions of the mu- 

 cous membrane and dissolution of bone produce the frontal, sphenoidal, and 

 maxillary sinuses.) Fibers of the olfactory nerve at first course between the 

 unjoined mesial and lateral masses. Later cartilaginous, and finally bony 

 trabeculae surround these bundles of nerve fibers, and, as the cribriform plates, 

 interconnect the three masses. 



Temporal Bone. — Several centers of ossification in the periotic capsule unite 



Tympankunf' 



Pcirosum 



Fig. 320. — The left temporal bone 

 at birth. The portion of intracarti- 

 laginous origin is represented in 

 stipple. 



