246 THE VERTEBRATE SKELETON. 



when taken together can in most vertebrates be seen to be 

 more or less arranged in three rings or segments called respec- 

 tively the occipital, parietal and frontal segments ; in the 

 Crocodile however only the occipital and parietal segments 

 are clearly seen. 



The occipital segment consists of four cartilage bones, 

 three of which together surround the foramen magnum. 



The most ventral of these, the basi-occipital (figs. 43 

 and 45, 9), forms the single convex occipital condyle for 

 articulation with the atlas, bounds the base of the foramen 

 magnum, and is continuous laterally with two larger bones, 

 the exoccipitals (fig. 45, 24), which meet one another dor- 

 sally and form the remainder of the boundary of the foramen 

 magnum. Each is drawn out externally into a strong process, 

 which is united below with the quadrate, and above with the 

 squamosal by a surface seen in a disarticulated skull to be 

 very rough and splintered. In a longitudinal section the 

 anterior face of the exoccipital is seen to be closely united 

 with the opisthotic. 



The exoccipital is pierced by a number of foramina, four 

 lying on the posterior surface. Just external to the foramen 

 magnum is a small foramen for the exit of the hypoglossal 

 nerve (figs. 44 and 45, XII). External to this is the foramen 

 for the pneumogastric (fig. 44, X), while more ventrally still 

 is the foramen (fig. 44, 15) through which the internal carotid 

 artery enters the skull. Some distance further to the side, 

 and more dorsally, is a larger foramen which gives passage 

 to the facial nerve and certain blood-vessels. 



In a median longitudinal section of the skull the hypo- 

 glossal foramen is seen, and just in front of it a small foramen 

 for a vein. Further forwards the long slit-like opening 

 between the exoccipital and opisthotic is the internal audi- 

 tory meatus (fig. 45, VIII) through which the auditory 

 nerve leaves the cranial cavity and enters the internal 

 ear. 



