THE ANIMAL CLIMAX-THE CHORDATES 



307 



oculomotor nerve 3 

 trochlear nerve 4 



/ 

 epiphysis 



Spinal nerve I 



externol nares 



olfactory nerve I 



abducens nerve 



/ 

 tympanic 

 membrane 



triqeminal nerve 5 and 

 facial nerve 7 



auditory nerve 8 



' •■ , 



qlossopbarynqeol nerve 9 ^^^^ 

 vaqus nersfe 10 



pHoctory lobe 

 lens of eye 



cerebrvjrn (telencephalon) 

 optic nerve 2 



diencepbalon 



mesencepbalon 

 optic lobe 



„ \ 

 cerebel lum (metencephalon) 



semicircvlar canal 



noedulla oblonqota 

 (njyelencepba I on) 



Fig. 13-31. Frog brain and cranial nerves, dorsal view. 



which make up the food-getting and breath- 

 ing mechanism. The jaws of the frog are 

 made up of three bones and resemble the 

 gill arches of tlie shark from which they 

 were derived. In man these bones have 

 fused into a single bone, the mandible 

 (Fig. 13-30). Behind the jaws and lying be- 

 tween them is the tongue, which possesses a 

 rather good support, the hyoid apparatus. 



The hyoid apparatus, like the jaw, has been 

 derived from the gill arches. Other gill 

 arches have been modified into supporting 

 structures for the larynx, the sound-making 

 apparatus. Here is seen the method con- 

 stantly used in evolution, that is, the forma- 

 tion of new structures from old ones which 

 no loncrer function in their original manner. 

 Nervous system. The nervous system of 



