206 Anatomy of Skeleton 



on the whole downwards and backwards and outwards ; each is raised from the surface 

 on a block of bone traversed by the anterior condylar canal, the lower opening of which 

 is above the antero-lateral margin of the condyle. A posterior condylar fossa and 

 foramen (not always present) are placed behind the condyle. 



The long axes of the condyles form an angle of 65 degrees with each other ; they 

 permit a nodding movement through an angle of about 45 degrees, and a certain amount 

 of lateral rocking, but no rotation is possible (see p. 18). Very rarely there is an articu- 

 lation in front for the atlas or odontoid ; the only condyle present in birds and reptiles 

 is an anterior median one. 



Looking into the foramen from above, the prominence of each condyle is more 

 striking, and above it is seen the upper opening of the anterior condylar canal : below 

 this is a small bony tubercle marking the attachment of the lateral occipito-odontoid 

 ligament. 



Basi-occiput : in front of the foramen, a thick, short, rod-like piece of bone. 



Lower aspect shows : 



(a) A pharyngeal tubercle for the attachment of the mid-point of the posterior 

 wall of the pharynx. 



(b) Markings behind and on each side of the tubercle for insertion of the 

 anterior Recti. 



(c) A small area in front of the tubercle in relation with mucous membrane 

 and pharyngeal tonsil (A. in Fig. 169). 



Upper aspect is grooved for reception of medulla and vertebral arteries, these 

 being supported by a cushion of subarachnoid fluid. Deep to these the cruciform 

 ligament and membrana tectoria are attached a little above the margin of the foramen, 

 where their markings may be found by the finger. Along each lateral border of this 

 aspect is a small half-groove for the inferior petrosal sinus, completed by the petrous 

 bone in the articulated skull. 



The exoccipital is that part of the bone that lies outside the foramen magnum 

 and forms all its margins except the extreme front and back parts ; it carries the hinder 

 three-quarters or so of the condyle, and the line of its junction with the basi-occipital 

 is marked by the anterior condylar canal. It forms the posterior wall of the jugular 

 foramen and has on its outer side a roughened articular surface that is separated from 

 the petrous bone by a layer of cartilage which ossifies in middle age. On the upper 

 aspect of its junction with the basi-occiput is seen (Fig. 169) a jugular tubercle and, 

 behind this, a groove for the ninth, tenth and eleventh nerves as they run to the jugular 

 foramen. 



The upper surface of the exoccipital has a prominent hooked process standing 

 up at its outer end, and round this the deeply-cut groove for the sigmoid sinus turns 

 sharply forward : the " posterior condylar canal " opens into the front and lower 

 part of this groove. The lower surface (Fig. 169) shows rough markings for Rectus 

 lateralis inserted here, and one of these ridges may be produced downwards as a par- 

 occipital process, which may even articulate with the atlas. 



Now articulate the bone with the temporal and look at the jugular foramen between 

 them ; it has a larger part externally for the vein and a small inner part for nerves, and 

 these two parts are often separated in part or wholly by a small bony projection from 

 the occipital above the anterior condylar foramen, called the " intra-jugular process." 



The tabular or squamous part can be said to consist of a post- or infra- occipital 

 and a supraoccipital portion : looking at the complete skull it can be seen that the part 

 immediately behind the foramen magnum looks down towards the neck and is marked 



