Chap. 34 AMPHIBIANS 693 



high-domed human one (Fig. 34.12). Between the cranium and upper jaw 

 are the capsules that hold the sense organs for smell, hearing, and sight. The 

 relatively enormous cavities for the eyes have no bony floor and, when the 

 eyelids are closed, the eyeballs bulge down into the mouth cavity, seeming 

 about to be swallowed. An opening in the posterior end of the cranium (fora- 

 men magnum) makes the cranial cavity continuous with the canal (neural 

 canal) in the vertebral column. The skull can be revolved only slightly on the 

 first and only vertebra of the neck. 



The vertebral column consists of nine vertebrae and the urostyle which func- 

 tions as a balance rod swung in the crotch of the pelvic girdle. It represents a 

 number of tail vertebrae now fused together and unrecognizable, but believed 

 to be the fused remains of the vertebrae of external tails in ancient amphibians. 

 The human skeleton also carries the remains of a once external tail that still 

 shows in the fused vertebrae of the coccyx. 



The pectoral girdle forms attachment places of the forelimbs and an almost 

 complete circlet of the body over the heart, lungs, and liver. It supports part 

 of the body's weight but is fastened to the vertebral column only by muscles 

 and ligaments which allow it to slide and act as a shock absorber in jump 

 landings. Dorsally, it consists of the flat shoulder blades (suprascapulas) that 



Nasal opening 



Ptialanges 

 Metacarpus — ' 

 Carpus (wrist) 



Metatarsus 



(Ankle 



Femur 



Tibio- tarsus 



Tarsus 



Premaxilla 



Maxilla (upper jaw) 



Orbit (eye) 



4^4 — Fronto parietal 

 (broin case) 



Atlas 



Scapula 

 7 Vertebrae 



Urostyle 

 Iliunn 



sctiium 



Fig. 34.12. Skeleton of the frog. 



