IV. VERTEBRATA 



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

 cartilaginous envelopes of the two sense organs, the nasa! and olic capsules, 

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

 eye which, although its 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 ifontanelles) occur in its roof, and frequently in its floor. The 

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

 nective tissue (priinordial 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. 



/? pf, 



/IS pro as 



Fig. 516. — Skull of carp, the visceral skeleton removed. (A) Cartilage bones: och, 

 ocl, ocs, basi-, ex-, and supraoccipitals; epo, epiotic; pto, pterotic; spho, sphenotic; pro, 

 prootic; as, alisphenoid; os, orbitosphenoid; me, mesethmoid; ee, ectethmoid. (P!) 

 Ventral membrane bones; px, parasphenoid: vo,\-omti. (C) Dorsal membrane bones: 

 p, parietal;/)-, 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 difl'erent 

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

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

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

 which, strictly speaking, do not belong to it. 



