IV. VERTEBRATA 



157 



end of the notochord. The side walls of the skull are increased by the 

 cartilaginous envelopes of the two sense organs, the nasal and otic capsules, 

 around the nose in front and ear behind. Between these is a hollow for the 

 eye which, althoughits capsule (sclera) may be cartilaginous or even ossified 

 in part, needs to be movable and hence it contributes nothing to the skull. 

 In only a few forms is the chondrocranium completely closed; usually 

 <;aps (fontanellcs) occur in its roof, and frequently in its tloor. The 

 higher the animal intellectually and the larger its brain the more the con- 

 nective tissue (primordial cranium') is called upon to roof in the chondro- 

 cranium. 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 membrane bones play 

 an important part in the completion of the skull. 



/? pfo 



ocs. 



we. 



FIG. 516. Skull of carp, the visceral skeleton removed. (A) Cartilage bones: ocb, 

 od, ocs, basi-, ex-, and supraoccipitals; epo, epiotic; pto, pterotic; spho, sphenptic; pro, 

 prootic; as, alisphenoid; os, orbitosphenpid; me, mesethmoid; ee, ectethmoid. 

 Ventral membrane bones: ps, parasphenoid: vo, vomer. (C) Dorsal membrane bones 

 P, parietal; fr, frontal; 1-4, exits of nerves. 



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 complicated arrangement 

 of the constituent bones. It may be said that in general the same bone 

 reappears in the different classes, and that the difficulties are connected 

 with the fact that certa'n bones may fail to develop (Amphibia), or they 

 may fuse to larger elements (mammals). A further complication results 

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

 which, strictly speaking, do not belong to it. 



