262 



ESSENTIALS OF ZOOLOGY 



bones. The occipitals are four in number. The supraoccipital 

 is placed at the back of the skull, and it forms the upper 

 margin of the foramen magnum. The exoccipitals are laterally 

 situated, and form the side margins of the foramen magnum. 

 They provide the occipital condyles for articulation with the 

 atlas, are pierced by the condyloid foramen for the twelfth 

 nerve, and are produced into the paramastoid process on each 

 side. The basioccipital completes the foramen magnum and 

 just contributes to the condyles. It extends forward like a 



Lachrymal 



Orlito- 

 sphejwid AlispTienoid 



Supraoccipital 



Nasal 



Premaxilla 



Condyle 



Paramastoid proc, 

 Ptrlotic 

 Tympanum 



FIG. 130. Skull of dog. External view. 



vertebral centrum to form the posterior element of the base of 

 the brain case. Laterally it is grooved for the internal jugular 

 vein, which enters the brain case through a wide opening 

 between the occipital, the tympanic and periotic. This open- 

 ing is called the foramen lacerum posterius and transmits 

 also the nerves IX, X, XI. 



The sphenoids are six in number, the median basi- and 

 pre-sphenoids and the two pairs of wings, the ali- and orbito- 

 sphenoids. The articulation between the basioccipital and 

 the basisphenoid is along a line joining the front surfaces of 

 the tympanic bones. From this line it extends forward, again 

 centrum-like, to meet the presphenoid. On the cranial surface 

 it bears a depression, the sella turcica, which lodges the pituitary 



