224 



BRANCH CHORDATA. 



excretions would also imply a sense of smell, but these may be 

 wholly for defense. The nostrils ^ of toads and frogs can be 



closed by special muscles. 



The lateral line of the tad- 

 pole disappears in the adult. It 

 seems that whatever senses may 

 be located in the lateral line, 

 they are ineffectual outside of 

 water. 



In most amphibians there is 

 an internal ear which opens by 

 one or two openings into the 

 mouth, back of the openings 

 from the nostrils. None of them 

 have any outside opening to the 

 ear, but most of the higher forms 

 (Anura) have a drum-cavity and 

 a tympanic membrane over it, 

 lying at the surface. A single 

 bone, the columella, lies across 

 the middle ear and has one 

 end against the tympanum. 

 There is no cochlea, or at least a 

 very rudimentary one; hence it 

 does not seem possible that the 

 frog can detect differences in 

 pitch. Perhaps this accounts for 

 his monotonous song. Yerkes 

 found that frogs "straightened 

 up and raised the head as if list- 

 ening when other frogs croaked 

 or splashed into the water," but 

 found it impossible to make them 

 res]5ond in any way to any noise 

 he himself made so long as he 

 remained invisible.^ He thinks 

 they depend on sight for the 



Fig. 185. — Brain and spinal cord 

 of frog (x about 2): a, Cerebral 

 hemisphere; b, olfactory lobe; c, 

 eye; d, thalamencephalon ; e, optic 

 lobes; f, cerebellum; g, medulla 

 oblongata; h, fourth ventricle; i, 

 spinal cord; I, Olfactory nerves; 

 II, optic nerve; III, oculomotor 

 nerve; IV, patheticus; V, fifth 

 nerve; VII, facial nerve; VIII, 

 auditory nerve; IX, glossopharyn- 

 geal nerve; X, vagus nerve; 1-10, 

 first to tenth spinal nerves; 2 and 

 3 unite to form the brachial, and 

 7, 8, and 9, to form the sciatic 

 plexus. (Shipley and McBride.) 



1" There seems to be no experimental proof of specific taste or smell 

 among amphibians or reptiles." — Washburn. ^ Linville and Kelly. 



