THE SKELETAL SYSTEM 15 



Bones of the Cranium 



Having examined the general topography of the skull, 

 the structure of each bone should be studied in greater 

 detail.* 



Occipital Bone (Os Occipitale) 



This bone first appears in the mammalian embryo as 

 four centers of ossification in the chondrocranium (carti- 

 laginous cranium). One of these centers is dorsal (supra- 

 occipital), one ventral (basioccipital), and two are lateral 

 (exoccipitals) to the foramen magnum. These four bones 

 fuse in mammals to form the occipital. The supra- and 

 ex-occipital regions in the rat are the posterior wall of 

 the cranium. The basioccipital is the floor of the cere- 

 bellar fossa. 



The basioccipital region is trapezoidal in shape, broader 

 behind than in front. Anteriorly it joins the basisphenoid 

 bone by a suture at right angles to the long axis of the 

 skull. Posteriorly it forms the ventral boundary of the 

 foramen magnum. Laterally it comes in contact with, but 

 is not fused to„ the tympanic bulla and the petrosal 

 bone. Note the two marked depressions on the ventral sur- 

 face of the bone which are separated from one another by 

 a median ridge. The inner surface of the basioccipital, 

 where it comes in contact with the brain, is concave. 



The occipital condyles and the triangular jugular proc- 

 esses previously mentioned, are parts of the exoccipital 

 regions. The condyles bound the foramen magnum ven- 

 trolaterally. They articulate with the atlas, or first neck 

 vertebra. This articulation permits the head to be raised 



* By boiling in water or an aqueous solution of sodium hydroxide 

 the bones may sometimes be separated along the sutures. Too pro- 

 longed boiling in the hydroxide solution will make the bones so 

 fragile that they will easily fall to pieces when handled. 



