26 COMPARATIVE ANATOMY 



(2.) The sense capsules, of which there are three pairs. The 

 olfactory capsules, which contain the organs of smell, are 

 situated anteriorly ; the auditory or otic capsules, containing 

 the organs of hearing, are situated one on each side of the 

 posterior region of the cranium. Both the auditory and 

 olfactory capsules are firmly united to, and continuous 

 with, the cranium. The optic capsules, comprising the 

 organs of sight, are never united to the cranium, but pro- 

 foundly modify that part of the skull in which they lie. 

 (3.) The jaws, present in nearly all craniate vertebrates, 

 comprise an upper and a lower jaw. The latter is always 

 articulated to the skull, and falls away from it in the dried 

 skeleton (except in the badger and sea-otter) ; the upper jaw 

 in the majority of vertebrata is firmly fastened to the cranium, 

 and appears to form part of it. 



The frog's head is large, flat, triangular in shape, with the 

 blunt apex forming the snout. But the cranium is relatively 

 small, the large size of the head being due to the wide sweep 

 of the jaw-bones and the large size of the cavities for the eyes. 



In the tadpole the brain case is entirely composed of 

 cartilage, but in the adult frog certain regions of the 

 cartilaginous case become ossified — turned into bone — but 

 the greater part remains cartilaginous throughout life. 



On either side of the great hole — the foramen magnum — at 

 the back of the cranium, is an ossification known as the 

 ex-occipital. The ex-occipitals are separated from one another 

 by a cartilaginous piece above and below in the middle line, 

 but elsewhere form the border of the foramen magnum, and 

 they bear two cartilaginous articular heads, the occipital 

 condyles, for articulation with the atlas vertebra. 



The roof, the sides, and the floor of the middle part of the 

 cranium are cartilaginous, but anteriorly it is completed by a 

 single bone, shaped somewhat like a dice box, one cavity of 

 which, the posterior, lodges the fore part of the brain ; the other, 

 divided into two by a median vertical partition, affords a passage 

 for the nerves of smell, and lodges part of the olfactory organs. 

 This bone, the sphenethmoid, is peculiar to Batrachia. 



The ex-occipitals and sphenethmoid are the only ossifications 

 of the cranium proper, but there is a pair of ossifications in 

 the ear-capsules which are continuous with, and scarcely to be 

 differentiated from, the cranial box. The ear-capsules are 



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