520 TEXTBOOK OF ZOOLOGY 



The lower jaw is formed primarily of a cartilaginous rod known as 

 Meckel's cartilage. At the extreme anterior tip of the jaw the rod is 

 ossified to form two small bones, the mentomeckelian bones. It is sub- 

 sequently covered anteriorly by a dentary bone and posteriorly by an 

 angulosplenial bone. The jaws are attached to the cranium by a com- 

 bination of three bones on each side, the squamosal, pterygoid, and 

 palatine, to form a suspensory mechanism. 



The vertebral column is made up of a series of nine typical verte- 

 brae and a long bone, the urostyle, which includes a fusion of the 

 vertebrae of the tadpole tail. 



In the neck region, there is one cervical vertebra, the atlas, which 

 articulates with the skull. This is followed by seven trunk vertebrae, 

 then one sacral vertebra whose processes support the pelvic girdle, 

 and finally the urostyle, which contains all of the caudal vertebrae 

 fused into one piece. 



The basal portion of the typical vertebra is known as the centrum. 

 The centrum is concave in front and convex posteriorly, and there- 

 fore is procoelous except one vertebra which is ampliicoelous in Rana. 

 Attached to the centrum is a bony arch, the neural arch, which ex- 

 tends dorsally from the centrum around the spinal cord. The neural 

 arch has extending from its sides, at the point of union with the 

 centrum, a pair of riblike transverse processes to which muscles are 

 attached. A dorsal projection of the neural arch is the neural spine. 

 In addition, the neural arch has at each its anterior border and pos- 

 terior border a pair of processes known as zygapophyses by which 

 the vertebrae are coupled together, the posterior zygapophyses 

 of one vertebra overlapping the anterior zygapophyses of the succeed- 

 ing one (Fig. 281). This arrangement furnishes a protected canal for 

 the spinal cord and a firm axial support which also allows bending of 

 the body. The spinal nerves emerge between vertebrae through 

 intervertebral foramina protected by the cartilaginous pads between 

 the vertebrae. 



Appendicular Skeleton. — The anterior portion of the appendicu- 

 lar skeleton is composed of the pectoral girdle, sternum, and bones 

 of the forelimbs. The posterior portion has the pelvic girdle and 

 bones of the hindlimbs. 



The pectoral girdle and sternum furnish a support and place of at- 

 tachment for the forelimbs and their muscles. They also provide a 

 case to protect the heart, lungs, and other organs in the anterior part 

 of the body. This girdle is not connected to the vertebral column. 



