CLASS AMPHIBIA 



519 



nium are the slender palatines extending laterally on each side 

 from the anterior end of the sphenethmoid to the upper jaw. The 

 vomers form the floor of the olfactory capsules, and their ventral 

 surfaces bear the vomerine teeth. The parasphenoid forms the 

 floor of the brain case. 



At the posterior end of the cranium is a large opening, the foramen 

 magnum, through which the spinal cord passes. On each side of this 

 opening are the exoccipital bones. Each bone has a rounded projec- 

 tion at its base, an occipital condyle, which articulates with the ver- 

 tebral column. 



Dorsal fissure ^ f\ Neural spine 



OraymatteK. \ | I , Neuralarch 



Zygapophysis ^^^^^_ ?^\^<^ W^C^ ^/^ y^ Tram'^erx process 



VJhibzmatten -^^^^"..^^^'^^^^^^^^^^'ssss^livW IXiramaker 



Dorsal root _ '^:i^£^!!!!!^^^^ ^::::::i^^ — Pla mater 



Ventral fissum. ^/^^^^^^^ i- \^ Ventral root 



I 

 Centrum 



Fig-. 281. — structure of a single vertebra and cross section of the spinal cord. 

 (Redrawn and modified from Holmes, Biology of the Frog, by permission of The 

 Macmillan Company, after Howe, Atlas of Zootomy.) 



Visceral Skeleton. — The visceral skeleton is that part of the axial 

 skeleton which consists of the jaws and hyoid apparatus in the 

 adult. The gill arches of the tadpoles are included in this portion. 

 These parts originate in cartilage which is later partially replaced 

 and reinforced by ossifications. The hyoid apparatus is primarily 

 cartilaginous and serves as a support for the base of the tongue and 

 the larynx. According to some authors, the jaws and the hyoid 

 were originally the branched arches supporting the gills, and evi- 

 dence of this is seen when the frog tadpole breathing with gills 

 transforms to the frog breathing without gills. 



The upper jaw consists of a pair of short premaxillary bones in 

 front, a pair of long maxillae fonning the sides, and a pair of short 

 quadratojugals as the posterior portions. The premaxillae and 

 maxillae each bear a row of small conical teeth. 



