THE CRANIUM. 



167 



Separate centres of ossification appear in the parachordal 

 cartilages to form (1) the basi-occipital, (2) the two exocci- 

 pitals, and (3) the supra-occipital. The occipital consists of 



$jS?-parach. cart/7. 



notochord 

 parach. cart/1 



alar proc. 



£3 — 1st ceruic. 

 ■2nd ceruic. 



Fig. 133. 



supra-occip. 



Fig. 134. 



Fig. 133. — The Parachordal Cartilages out of which the Cartilaginous Parts of the 

 Occipital Bone are formed. 



Fig. 134. — The expansion backwards of the Parachordal Cartilages to enclose the 

 Foramen Magnum and form the Supra-occipital. 



these four pieces until the fourth year, when synostosis occurs. 

 The occipital condyles are formed from the exoccipitals and basi- 

 occipital, the exoccipital element constituting by far the larger part. 

 The anterior condylar foramen is formed between these two parts. 

 The occipital protuberance is formed by both membranous and 

 cartilaginous parts of the supra-occipital. 



(2) The Petro-mastoid forms part of the base of the skull. We 

 have already seen that the petro-mastoid part of the temporal 

 bone is developed out of the cartilage which forms the periotic 

 capsule (Figs. 35, p. 50, and 135). The periotic cartilages fuse 

 at points with the parachordal, which form the basis of the 

 occipital bone. Even to a late stage (30th year or later) 

 remnants of these cartilages may be found between the petro- 

 mastoid and occipital bones, especially between the jugular 

 process of the occipital and the mastoid (Fig. 136). The fibro- 

 cartilage in the foramen lacerum medium is a remnant of the 

 periotic cartilaginous capsule. (See also p. 58.) 



