BONES OF THE SKULL. 



the petrous temporal. A fissure (petro-occipital) runs back- 

 ward from the foramen, between the basi-occipital and petrous 

 temporal; it is closed by cartilage and opens behind into an 

 irregular orifice (jugular foramen) between the petrous tem- 

 poral and jugular process of the occipital. A projection 

 (processus intra-jugulare) from the jugular process may divide 

 the jugular foramen. The jugular fossa is the excavated part 

 of the foramen which is formed by the petrous bone; in its 

 outer part, or its outer border, is a foramen (for Arnold's 

 nerve) ; the lower orifice of the carotid canal lies just in front 

 of it, separated by a ridge which presents a foramen (for 

 Jacobson's nerve). [164] 



External to the jugular foramen is the styloid process, en- 

 closed by the vaginal process (see page 8) ; the foramen (stylo- 

 mastoid) of the aqueductus Fallopii is just behind the root of 

 the styloid process. At the base of the inner surface of the 

 mastoid process is a deep (Digastric) groove, and just internal 

 to this is a shallow groove for the occipital artery. A stout 

 bar (jugular process) extends outward from the back part of 

 each occipital condyle to the petrous temporal. The condyles 

 are oval, flat transversely but convex antero-posteriorly, and 

 bound the sides of the anterior half of the foramen magnum; 

 they end anteriorly in rounded thickenings. Just external to 

 the front part of each condyle is a fossa (anterior condylic) 

 which is pierced posteriorly by a foramen (anterior condylic) ; 

 just behind each condyle is another fossa (posterior condylic) 

 which may be pierced by a foramen (posterior condylic). The 

 foramen magnum is large and oval; its extreme anterior and 

 posterior edges are the basion and opisthion. [165] 



The nuchal surface of the tabular part of the occipital lies 

 behind the foramen magnum; it is limited posteriorly by a 

 ridge (superior curved line) running transversely outward on 

 each side from a central eminence (external occipital protu- 

 berance). A mesial ridge (external occipital crest) descends 

 from the protuberance to the foramen magnum ; from its centre 



[13] 



