THE FROG 27 



Skull. 



The skull of the frog, as of all higher vertebrates, consists 

 of a case for the brain, the cranium, with which are connected the 

 sense capsules, the jaws and the hyoid apparatus. The sense 

 capsules comprise three pairs : the auditory, lodging the organ of 

 hearingand attached to the hinder end of the cranium ; the olfactory, 

 lodging the organ of smell and attached to the anterior end ; and 

 lastly, the optic capsule or eye, between the former two. In the 

 frog the optic capsule is not attached to the skull, but the other 

 bones are so arranged that a space, the orbit, is left for it. Two jaws 

 are present, the upper, firmly, although indirectly, attached to the 

 cranium, and consequently not capable of independent movement, 

 and the lower, connected with the upper by means of a joint at the 

 hinder end, and free to move in a vertical direction. The hyoid 



po 



FIG. 4. The skull of Ran a. A, Dorsal aspect ; B, Ventral aspect . 



e.o,, exoccipital ; f.m., foramen magnum ; f.o. fenestra ovalis ; /.p., fronto-parietal ; mx.> 

 maxilla ; na., nasal ; o.f., optic foramen ; pal., palatine ; par., parietal ; p.mx., pre-maxilla 

 P.O., pro-otic ; pt. pterygoid ; qu., quadrato-jugal ; s.m., septo-maxillary ; sp. sphenethmoid ; 

 si)., squamosal ; vo., vomer ; v.t., vomerine teeth. 



apparatus consists of the flat hyoid plate, situated as already noted, 

 in the floor of the mouth, and its projecting horns. 



In the middle of the back of the skull is a large hole, the 

 foramen magnum, through which brain and spinal cord are con- 

 tinuous. On each side of this, and forming its entire margin, save 

 for small pieces of cartilage in the mid-dorsal and ventral lines, is a 

 cartilage bone, the exoccipital, which is produced backwards into a 

 rounded knob, the occipital condyle. The condyle fits into and 

 articulates with the corresponding concavity on the front of the 

 atlas vertebra. If the skull be placed in its natural position with 

 regard to the atlas, it will be seen that on the dorsal surface a con 

 siderable space is left between it and the neural arch of the atlas. 



