IV. VERTEBEATA. 



521 



closed; usually gaps (fontanelles) occur in its roof, and frequently 

 in its floor. The higher the animal intellectually and the larger 

 its brain the more the connective tissue (primordial cranium) is 

 called upon to roof in the chondrocranium. Hence it is that in 

 the reptiles, birds, and mammals, where it is also confined to 

 embryonic life, the chondrocranium is relatively the smallest. 

 Since it only closes above in the occipital (hinder) region, while 

 it gaps widely in front, it follows that the secondary bones play an 

 important part in the completion of the skull. 



The bony skull presents great difficulties from the standpoint 

 of comparative anatomy, in part from its varying appearance in 

 the different groups, in part on account of the number and com- 

 plicated arrangement of the constituent bones. It may be said in 

 beginning that as a rule the same bone reappears in the separate 

 classes, and that the difficulties are connected with the fact that 

 certain bones may fail to develop (Amphibia), or they may fuse 

 to larger elements (mammals). A further complication results 

 from the intimate union with the cranium of bones of the visceral 

 arches, which, strictly speaking, do not belong to it. 



me. 



fis firo as 



FIG. 560. Skull of carp, the visceral skeleton removed. (A) Cartilage bones: ocb 

 ocl, ocs, basi-, ex-, and supraoccipitals ; ego, epiotic ; pto, pterotic ; sp/io, sphe- 

 notic; pro, prootic ; as, alisphenoid ; o.s, orbitosphenoid; me, mesethmoid ; ee, ect- 

 ethmoid. (B) Ventral membrane bones : p.s, parasphenoid ; vo, vbmer. (C) Dorsal 

 membrane bones : p, parietal ; /?-, frontal ; l-U, exits of nerves. 



The primary bones (preformed in cartilage) can be divided ac- 

 cording to the cranial regions into four groups: (1) bones of the 

 hinder part of the head occipitalia; (2) bones of the ear region 

 otica; (3) bones near the eye splienoidalia; and (4) of the 

 nasal capsule ethmoidalia. The occipitalia four in number 

 (figs. 560-562) united in the higher mammals to a single occipital 



