154 THE VERTEBRATE SKELETON. 



B. THE SKULL 1 . 



The skull of the Frog consists of three principal parts : 



(1) an axial part, the cranium, proper, which encloses 

 the brain. To it are firmly fused 



(2) the capsules of the olfactory and auditory 

 sense organs, 



(3) lastly there is the hyoid apparatus and the skele- 

 ton of the jaws. 



The skull is by no means so completely ossified as is the 

 vertebral column, but in addition to the cartilage bone, there 

 is a great development of membrane bone in connection 

 with it. 



The skull has a peculiarly flattened and expanded form 

 depending on the wide lateral separation of the jaws from the 

 cranium. 



(1) THE CRANIUM PROPER or Brain case. 



This is an unsegmented tube, which is widest behind. It 

 remains to a considerable extent cartilaginous, but is partly 

 converted into cartilage bone, partly sheathed in membrane 

 bone. Its roof is imperfect, being pierced by three holes 

 or fontanelles, one large anterior fontanelle (fig. 25, A, 9), 

 and two smaller posterior fontanelles (fig. 25, A, 10). 



The cartilage bones of the cranium proper are the two 

 exoccipitals and the sphenethmoid. 



The exoccipitals (figs. 24, 25, and 26, 6) are a pair of 

 irregular bones bounding the foramen magnum at the posterior 

 end of the skull. They almost completely surround the foramen 

 magnum, and bear a pair of oval convex surfaces, the occipital 

 condyles, with which the first vertebra articulates. The bones 

 generally called the exoccipitals of the frog include the epi-otic 



1 W. K. Parker, Phil. Trans. 161, 1871, p. 137, and W. K. Parker and 

 G. T. Bettany, The Morphology of the Skull, London, 1877, p. 136. 



