13 



parasphenoid and in the latter partly by the prootic and 

 partly by the parasphenoid. In both cases there is an 

 obvious space between the false and true floors, and this 

 space is the eye muscle canal. At the region of this 

 posterior bridge the side walls of the cranium are greatly 

 strengthened internally (and the cranial cavity hence 

 reduced) by a stout ridge of bone borne on the prootic, 

 sphenotic and supraoccipital. From the middle of this 

 ridge there extends backwards another process which be- 

 comes larger and more complex as it passes backwards. 

 This is formed mostly by the sphenotic, supraoccipital, 

 pterotic, epiotic and exoccipital, and consists of both bone 

 and cartilage. It is here that the cranial wall is thickest. 

 The foramen magnum does not open at once into the 

 cranial cavity, but into a bony canal formed by the 

 basioccipital and exoccipitals (fig. 4). 



Basioccipital {B.O., figs. 2, 3, 4). — A stout bone, 

 partly cartilaginous in front, and bearing the single con- 

 cave occipital condyle for the centrum of the atlas. Above 

 it forms a small portion of the floor of the foramen 

 magnum. Mid-ventrally it exhibits a deep depression 

 into which fits the posterior extremity of the parasphenoid. 

 The basioccipital is bounded above by the exoccipitals, 

 and laterally by the prootics, opisthotics and exoccipitals. 



Exoccipital {Ex.0., figs. 2, 3, 4). — Forms most of the 

 occipital foramen or foramen magnum (fig. 4, F.M.). It- 

 is not completely ossified, and above its cartilage forms 

 part of the cross-shaped wedge of cartilage appearing on 

 the surface of the occiput (fig. 4). Each exoccipital bears 

 a very prominent ridge and concave facet lined with car- 

 tilage for the corresponding process on the atlas. The 

 asymmetry of these paroccipital condyles {P.C.) has been 

 elsewhere noticed. The exoccipital is bounded above by 

 the epiotic, laterally by the pterotic and opisthotic, below 



