18 
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 (2.0., 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 (Hwv.0., figs. 2, 3, 4)—Forms most of the 
occipital foramen or foramen magnum (fig. 4, /’.d/.). 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 
