THE FROG 27 



placed one on either side of the posterior region of the 

 cranium, where they form prominent projections. The 

 anterior and lower face of each projection presents an ossifica- 

 tion known as the pro-otic bone. 



Anteriorly, the cartilage of the cranium is continued directly 

 into the cartilage of the olfactory chambers or nose ; so we see 

 that the central portion of the frog's skull is a cartilaginous 

 brain-box, with which the cartilaginous olfactory capsules are 

 continuous in front, and the auditory capsules at the side 

 and behind, and the cartilage is replaced by bone in the 

 ex-occipital, the sphenethmoid, and the pro-otic regions. But 

 the brain-box and sense organs are further protected by bones 

 above and below, which are not ossifications in cartilage, but 

 are formed in membrane as sub-dermal ossifications in the 

 integument covering the head. These bones over- or under- 

 lie the cartilaginous cranium, and can easily be stripped off 

 it, whereas the ossifications in cartilage cannot be stripped off 

 Above, running from the occipital or posterior region of the 

 skull as far forwards as the sphenethmoid, is a pair of bones 

 called the fronto-parietals. In front of these a pair of 

 triangular nasals roof over the cartilage of the chambers of the 

 nose. Below, the whole floor of the cranium is protected by 

 a large dagger-shaped bone, the cross-pieces of the handle 

 reaching beneath the auditory capsules. This is the 

 parasphenoid. The floor of the olfactory region is occupied 

 by a pair of small irregularly-shaped bones, the vomers, which 

 bear each a patch of teeth. 



The broad triangular skull is completed by the jaws 

 and the suspensory apparatus by which the jaws are 

 attached to the cranium. The gape of the upper jaw 

 is bordered on either side, by a pre-mazilla in front and 

 a slender maxilla behind, both bearing teeth. These two 

 bones are ossifications in the sub-dermal membrane and 

 therefore are membrane bones. The maxilla is attached to 

 the cranium before and behind by bony and cartilaginous 

 struts. From the ethmoid region of the skull, just in front 

 of the sphenethmoid bone, a bar of cartilage runs out on 

 either side. This bar is covered below by a slender bone, 

 the palatine. It is usual to reckon the palatine among the 

 "cartilage bones "of the skull because in most animals it is 

 preformed in cartilage. But, as a matter of fact, in the frog it 



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