THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 



195 



In the sphenoidal region ossification begins at a number of centers which, 

 as in the occipital region, correspond generally to the parts of the adult sphenoid 

 bone (Fig. 176). (1 and 2) About the ninth week an ossification center 

 appears on each side in the cartilage which corresponds to the ala magna 

 (alisphenoid). (3 and 4) About the twelfth week a center appears on each 

 side which corresponds to the ala parva (orbitosphenoid). (5 and 6) A 

 short time after this a center appears on each side of the medial line in the 

 basal part of the cartilage, and the two centers subsequently fuse to produce the 

 corpus (basisphenoid). (7 and 8) Lateral to each basal center, another center 

 appears which represents the beginning of the lingula. (9 and 10) Finally 

 two centers appear in the basal part of the cartilage, in front of the other 

 basal centers, and then fuse to form the presphenoid. As in the case of the oc- 

 cipital bone, not all of the adult sphenoid is of intracartilaginous origin; for the 



Ala parva- 



Ala magna 



Lingula 



Fig. 176. — Sphenoid bone of embryo of 3J-4 months. Sappey. 

 The parts that are still cartilaginous are represented in black. 



Corpus 

 (basisphenoid) 



upper anterior angle of each ala magna is of intramembranous origin, as are also 

 the medial and lateral laminae of the pterygoid process. The pterygoid hamulus, 

 however, is formed by the ossification of a small piece of cartilage which de- 

 velops on the tip of the medial lamina. The fusion of these various parts oc- 

 curs at different times. The lateral pterygoid lamina unites with the alisphe- 

 noid before the sixth month of foetal life; about the sixth month the lingula fuses 

 with the basisphenoid, and the presphenoid with the orbitosphenoid. The 

 alisphenoid and medial pterygoid lamina fuse with the rest of the bone during 

 the first year after birth. The union of the basisphenoid and basioccipital 

 usually occurs when the growth of the individual ceases, though the two bones 

 may remain separate throughout life. 



In the region of the periotic capsule, several centers t>f ossification appear in 

 the cartilage during the fifth month. During the sixth month these centers 

 unite to form a single center which then gradually increases to form the pars 

 pelrosa and pars mastoidea of the adult temporal bone. The mastoid process is 



