. A manual of zoology. Zoology. 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
. A manual of zoology. Zoology. 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
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
