THE AXIAL SKELETON 



313 



(inter parietal) part above that line is an addition of intramembranous 

 origin. These several components do not fuse completely until about the 

 seventh year. 



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

 corresponds to this bone (Fig. 318): (i and 2) in each a/a magna (ali- 

 sphenoid) ; (3 and 4) in each a/a parva (orbitosphenoid) ; (5 and 6) in the 

 corpus between the alae magnae (basisphenoid) ; (7 and 8) in each lingula; 

 (9 and 10) in the corpus between the alae parvae (presphenoid). Intra- 

 membranous bone also enters into its composition forming the orbital, 

 and temporal portion of each ala magna and the mesial laminae of each 

 pterygoid process (except the hamulus). Fusion of the various parts is 

 completed during the first year. 



Ala magna Ala parva 



(Alisphenoid) Presphenoid (Orbitosphenoid) 



. 



Lingula -'fl ', ^-Pterygoid 



Basisphenoid process 



PIG. 318. Sphenoid bone of a human fetus 

 of nearly four months (after Sappey). Parts 

 still cartilaginous are represented in stipple. 



Nasal septum 



Perpendicular plate 

 ta galli 



Cribriform plate 

 I Labyrinth 



PIG. 319. Ethmoid bone of a human fetus of 

 four months (modified after Kollmann). 



The 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 ot the 

 mesial mass persists as the cartilaginous nasal septum but ossification of 

 the upper portion produces the lamina perpendicularis and the crista 

 galli (Fig. 319). The lateral masses ossify at first into the spongy bone of 

 the ethemoidal labyrinths. From this, the definitive honeycomb struc- 

 ture (ethmoidal cells) and the conchas are formed through evaginations of 

 the nasal mucous membrane and the coincident resorption of bone. 

 (Similar invasions of the mucous membrane and dissolution of bonejjro- 

 duce the frontal, sphenoidal, and maxillary sinuses; p. 376). 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. 



The Temporal Bone. Several centers of ossification in the periotic 

 capsule unite to form a single center from which the whole cartilage is 



