224 THE VERTEBRATE SKELETON. 



Like those of the skull as a whole its component bones may 

 be subdivided into three sets : 



1 . those forming the brain-case or cranium proper ; 



2. those developed in connection with the special sense 

 organs ; 



3. those forming the upper jaw and suspensorial appa- 

 ratus. 



Both cartilage and membrane bones take part in the forma- 

 tion of the skull, and a considerable amount of cartilage 

 remains unossified, especially in the ethmoidal and sphenoidal 

 regions. 



1. The CRANIUM PROPER OR BRAIN-CASE. 



The cartilage and membrane bones of the brain-case when 

 taken together can be seen to be more or less arranged in 

 three rings or segments, called respectively the occipital, 

 parietal, and frontal segments. 



The occipital segment is the most posterior of these, and 

 consists of four cartilage bones, the basi-occipital, the two 

 exoccipitals and the supra-occipital; these bound the 

 foramen magnum. 



The basi-occipital (figs. 38 and 39, 5) lies ventral to the 

 foramen magnum and only bounds a very small part of it ; it 

 forms one-third of the occipital condyle by which the skull 

 articulates with the atlas vertebra. It unites dorsally with 

 the exoccipitals and anteriorly with the basisphenoid. 



The exoccipitals are rather small bones, which form the 

 sides and the greater part of the floor of the foramen magnum, 

 and two : thirds of the occipital condyle. Laterally each is 

 united with the pterygoid and opisthotic of its side. At the 

 sides of the occipital condyle each exoccipital is pierced by 

 a pair of foramina, the more dorsal and posterior of which 

 transmits the hypoglossal nerve. 



The supra-occipital (fig. 39, 14) is a larger bone than the 

 others of the occipital segment. It forms the upper border 



