56 The Vertebrate Organ Systems 



the eyes and the capsules that contain the ears. Anterior to the cranium 

 are small capsules that protect the nose. 



The roof of the cranium is composed of the frontoparietal bones; 

 lateral and posterior to these are the prootic bones that contain the 

 ears. The ventral surface is formed by the central parasphenoids and 

 the vomers which bear the vomerine teeth. The posterior wall of 

 the nasal cavity is formed by the sphenethmoid. Protecting the nasal 

 cavity dorsally are the nasals. 



PRCMAXILLA 

 NASAL 



SPHENETHMOID 



rnoNTOPABirTAL 



SOUAUOSAL 



EXOCCIPITAL 

 ATLAS 



SUPRASCAPULA 



PHALANOES 



_METACA«PALS 

 ARPALS 



CALCANEUS 



Fig. 11. — Dorsal view of skeleton of bullfrog. 



Just posterior to the prootics are the exoccipitals. Each of these 

 bears a rounded posterior projection, the occipital condyle, which 

 fits into a depression in the first vertebra. This arrangement allows 

 movements of the frog's head. Between the occipital condyles is a 

 large opening, the foramen magnum. Through this opening, the pos- 

 terior portion of the brain passes into the spinal cord. 



The jaws, which in the adult are part of the skull, have their 

 origin from other embryological structures. In the embryo, these 

 were formed as arches in the pharyngeal region. They are known as 

 the visceral skeleton because of this origin. The entire visceral skele- 

 tdn in the adult frog consists of the jaws, cartilages of the larynx, 

 the ear bones, and the hyoid arch which supports the tongue. The 



