A FROG 197 



The two nasal capsules lie side by side, fused with the anterior 

 end of the cranium proper, and are also composed largely of car- 

 tilage. The ringlike ethmoid bone, which, as we have seen, forms 

 the anterior end of the brain case, also forms the posterior end 

 of the nasal capsules. Two pairs of membrane bones are present 

 in these capsules, the dorsal nasals and the ventral vomers. The 

 former are a pair of large bones which lie in a transverse position 

 covering the cartilage just in front of the ethmoid ; the latter are 

 a pair of bones also in front of the ethmoid, on the ventral surface 

 of the skull, each bone bearing a group of small teeth. 



The upper jaw and the other remaining portions of the visceral 

 skeleton still remain to be described. The upper jaw forms two 

 distinct arches, an outer, or maxillary, arch, and an inner, or pala- 

 topterygoid, arch. 



The three bones of the maxillary arch, on each side, are the 

 quadratojugal, the small posterior bone ; the maxillary, the long, 

 thin bone which bears most of the teeth ; and the premaxillary, 

 the small anterior bone which forms the anterior end of the skull. 



The two bones of the palatopterygoid arch, on each side, are 

 the pterygoid and the palatine. The former is a large bone which 

 lies at the hinder end of the skull, medial to the quadratojugal 

 and the maxillary, and is best seen on the ventral surface. The 

 palatine is a slender bone which lies on the ventral surface and 

 extends from the maxillary at the forward end of the pterygoid 

 transversely to the ethmoid. 



The bones and cartilages by which the lower jaw is suspended 

 from the cranium are called the suspensorium. In the frog it is 

 formed on each side by a small cartilage, the quadrate, and a bone, 

 the squamosal. The quadrate Hes at the extreme lateroposterior 

 end of the skull, in close connection with the quadratojugal bone 

 and between the pterygoid and the squamosal ; the lower jaw ar- 

 ticulates with its outer surface. The squamosal is a T-shaped 

 bone which lies on the dorsal surface of the skull in the region 

 of the auditory capsule ; it supports the tympanum. 



Exercise 29. Draw a view of the dorsal aspect of the skull. Show 

 accurately the outlines of the bones and cartilages. 



Exercise 30. Draw a similar view of the ventral aspect. 



