130 



STRUCTURE OF THE VERTEBRATES 



3. The anterior sphenoid complex is in contact with the former 

 group, and like it has three bones: the unpaired presphenoid and 

 the lateral orhito sphenoids. The latter are at the level of the 

 optic capsule and form part of the orbit, or cavity, in which the 

 eye rests. 



4. The ethmoid group is most anterior, though covered by 

 dermal bones. These bones are developed in part from the an- 

 terior half of the prechordal cartilages, and partly from the 



Olf. cap. 



Foramen 

 7nagnmn 



Sijnotic tectum 



For. mag. 



Fig. 66, Chondrocraniiim of the 

 Pig. Centers of ossification appear 

 in the chondrocraniiim soon after 

 the first strands of bone appear in 

 the dermal fibers. (After Mead). 



Fig. 67. Ossification of the Chon- 

 drocraniiim (Diagrammatic). The 

 major regions are heavily outHned. 



olfactory capsule. The ethmoid bones of the lower vertebrates 

 are not as separate as in the mammals. In the latter group there 

 is a ventral ethmoid; a median mesethmoid, forming a septum 

 between the two nostrils; and the paired ectethmoids. 



The otic capsule ossifies as five separate centers. In the lower 

 vertebrates these are often separate. The lower mammals also 

 have these otic bones defined, but there is a tendency toward 

 fusion in the higher groups. In man the five embryonic bones 

 form a single petrosal bone which fuses with a dermal bone to 

 make the temporal. 



