488 



Cotnparative Animal Physiology 



not function below 32 cycles. The mechanoreceptors of the skin are ap- 

 parently as important as the labyrinth. Ferhat-Akat also demonstrated that 

 Amhlystoma is able to distinguish between frequencies that differ in pitch 

 as much as a miisical interval of a fourth or a fifth (frequency ratios of 

 132 : 1 and 1.50 : 1). In a few experiments on Salamander maculosa 

 larvae he showed that the upper limit of reaction was 1035 cycles. 



Frogs are relatively unresponsive to sounds, but they may have an acute 

 sense of hearing. Yerkes,"*' on the basis of a study of conditioned reflexes, 

 reported that frogs can hear sounds between 50 and 10,000 cycles, but he did 

 not obtain evidence of discrimination. Yerkes' frogs continued to respond 



endylymph duct 



posterior canal 



brain • 



anterior canal 

 inner ear 

 lateral canal 



tympanic membrane 



annular ring 

 middle ear 



Eustachian tube 



sacculus 



Fig. 165. Diagrammatic presentation of the ear of the frog, as seen from the posterior. 



From Adams.^ 



after removal of the ear drum and columella but not after cutting of the 

 auditory nerves. Adrian, Craik, and Sturdy'* studied the microphonic effect 

 in frogs and obtained responses between 50 and 300 cycles, with the maxi- 

 mum sensitivity between 50 and 200. Ross""'^ in a study of single nerve fiber 

 preparations observed a response to vibrations of about 100 cycles per second 

 but did not investigate the extent of the range. 



Witschi''*^ demonstrated that during the early development of tadpoles the 

 round window of the inner ear becomes connected with a bulge of the lung 

 sac by means of a fibrous cord. This cord t)ecomes cartilaginous, pierces the 

 dorsal aorta, and serves as an aortic columella which transmits vibrations to 

 the inner ear. In this respect the aortic columella functions in the same way 

 as the Weberian ossicles of fishes. The oval window serves as a pressure re- 

 lease mechanism. During later development the aortic columella disappears 

 and is replaced by the tympanic columella, which develops in the middle ear 

 and makes contact with the oval window. The round window then serves 

 as a pressure release, as in higher forms. 



