42 THE FROG CHAP. 



In the disarticulated skull it can be made out that the 

 exoccipital and pro-otic of either side enclose a cavity ; in 

 this the organ of hearing is contained. The exoccipital is 

 perforated, just in front of the condyle, by a double aperture 

 (Nv. g, 10), through which two nerves, the glossopharyngeal 

 and the vagus, pass on their way from the brain (see 

 Chapter X.) . The pro-otic is similarly perforated or notched 

 for the trigeminal and facial nerves (Nv. 5, 7). 



The dorsal surface of the brain-case is covered by two 

 longish, flat bones (FR.PA). In the young condition 

 each of these consists of two distinct bones, the front one 

 the frontal, the hinder the parietal. As the young frog 

 grows the frontal and parietal of either side become com- 

 pletely fused, forming a single fronto-parietal. On the upper 

 surface of each olfactory capsule is a roughly triangular bone, 

 the nasal (NA ) , in front of which is the corresponding nostril. 



The ventral surface of the brain-case is covered by a 

 single bone (PA.SPH) having the shape of a T. The 

 stem extends forwards in the middle line as far as the 

 olfactory capsules, while the arms stretch outwards beneath 

 the auditory capsules. This very characteristic bone is 

 the parasphenoid. On the under surface of the olfactory 

 capsules, corresponding to the nasals above, are a pair of 

 irregular bones, the vomers ( VO). Their outer edges are 

 notched and help to bound the internal nostrils : their 

 posterior ends bear the vomerine teeth. 



The anterior end of the brain-case is surrounded by a 

 bone (SP.ETH) which extends forwards into the region of 

 the olfactory capsules, and is partly covered by the fronto- 

 parietals and nasals above and by the parasphenoid below. 

 This is the girdle-bone or sphenethmoid. In the disarti- 

 culated skull it is seen to have a very peculiar shape. Its 

 posterior half encloses a single cavity in which the fore-end 

 of the brain (Fig. 6, olf. I) is lodged. Its anterior half 

 encloses two cavities, -right and left, separated from one 

 another by a vertical partition, and serving to lodge the 

 posterior ends of the olfactory sacs or organs of smell. Each 

 of these cavities communicates with the single posterior 

 cavity by a small hole through which the nerve of smell 

 passes. 



Between the girdle-bone in front and the pro-otic behind, 

 the side-walls of the skull are formed of cartilage perforated 

 by a rounded aperture, the optic foramen (Nv. 2), for the 

 nerve of sight. 



Forming the outer part of the suspensorium is a hammer- 

 shaped bone, the squamosal (SQ) ; its head is applied to the 

 auditory capsule and projects forwards into the orbit. 



