BULLFROG AS TYPICAL VERTEBRATE ANIMAL 83 



Skeletal System 



The bullfrog has no exoskeleton, its body being covered by 

 smooth skin. The endoskeleton may be considered in two main divi- 

 sions, the axial and appendicular portions. The axial part includes 

 the skull and vertebral column; the appendicular portion consists of 

 the bones of the limbs and their supports, the pectoral and pelvic 

 girdles. 



Bones are joined to one another by structures made up of connec- 

 tive tissue which allow varying degrees of movement between them. 

 These structures are called joints or articulations. In some cases, as 

 in the skull, the joints are immovable and the bones are separated only 

 by a thin sutural ligament of connective tissue. In other cases, the 

 joints are slightly movable, as in the vertebral column where a plate 

 of dense tissue and cartilage connect the vertebrae. In still other 

 cases the bones are freely movable, as in the limbs,. and here the bones 

 are entirely separated, but are held in place by ligaments. 



The Axial Skeleton. — The skull, which is pomposed of cartilage, 

 cartilage bones, and membrane bones, forms a case for the brain and 

 capsules for the sense organs. Cartilage bones are the sphenethmoids, 

 pro-otics, exoccipitals, pterygoids, palatines, and cartilaginous quad- 

 rates. The membrane bones develop from ossifications of membranes 

 which cover the cartilage and cartilage bones. They are thin and 

 may be separated from the others. The membrane bones are the 

 premaxillaries, maxillaries, nasals, frontoparietals, quadratojugals, 

 squamosals, parasphenoids, and vomers. The bone* enclosing the 

 brain constitute the cranium. 



On the dorsal surface of the cranium, the two frontoparietals 

 form most of the roof, the pro-otics form the roof of the auditory 

 capsule (inner ear capsule), the sphenethmoids form the posterior 

 wall of the olfactory capsule (nasal chamber), and the two tri- 

 angular nasal bones lie above. On the ventral surface of the cra- 

 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 



