IX.] OP THE DOG. 99 



side to side, and forming the floor of the cranial cavity; 

 but the continuation of the axis forward is of a different 

 character. The anterior end of the presphenoid narrows 

 considerably, and the segment in front of it, in very young 

 skulls, is a much compressed vertical plate of cartilage, of 

 very considerable size, both from before backwards and 

 from above downwards. Ossification of this cartilage com- 

 mences in the posterior end and upper part, and spreads 

 forwards and downwards, but it never or very rarely reaches 

 its anterior extremity ; and in the animal now described a 

 narrow inferior margin remains permanently cartilaginous. 

 The ossified portion of this cartilage {ME) constitutes the 

 lamina perpefidicularis of the ethmoid bone, the anterior 

 unossified portion the septal cartilage of the nose, which is 

 the anterior termination of the cranio-facial axis. The 

 term mesethmoid may be applied to the whole of this element 

 of the skull, whether ossified or not. 



Above all the posterior, or basicranial, part of this axis, 

 constituted by the three first-mentioned bones, is the cere- 

 bral cavity, the walls of which constitute the " brain-case." 



These walls are formed by several more or less expanded 

 and curved bones, which rise up from the sides of the axis 

 or floor of the cavity below, and, meeting in the middle line, 

 roof in the cavity above. These bones are arranged in 

 three sets from behind forwards, each corresponding with 

 one of the axial bones, and with the latter constituting one 

 of the three segments or bony rings into which the brain- 

 case may be divided. 



The hindermost (or occipital) segment consists of the 

 basioccipital below; next on each side the exoccipitals(EO), 

 and a large, median, flat bone above, with its upper ex- 

 tremity prolonged forwards in the middle line between the 

 bones of the next segment, called the supraoccipital {$0)* 



H 2 



