Phonoreception 



483 



ences in the two ears may be as much as 30 db, thereby contributing materially 

 to the problem of localization. A phase difference means that the sound 

 pressure reaches a maximum at one ear shortly before it does at the other ear. 

 This should give rise to time differences of nerve impulses in corresponding 

 nerve fibers from the two ears, especially for low frequency sounds, i.e., below 

 800 cycles. Some individuals are very sensitive to this "phase effect," and 

 others are apparently insensitive. In general, for localization the phase ef- 

 fect seems to be more important at low frequencies, and the intensity effect 

 at high. 



PHONORECEPTION BY OTHER VERTEBRATES 



In lower vertebrates the hearing mechanism is not nearly so well devel- 

 oped as it is in mammals. From a very slight development in certain fishes 

 it is possible to trace phylogenetic series showing the development of the audi- 

 tory organ (Fig. 161). The sacculus and semicircular canals are found in 

 cyclostomes and in all higher chordates. Hagfishes, however, have only one 



Utriculus 

 Socculus 



Utriculus ' [^' 



Lagena Sacculus 



Papilla 

 basilaris 



Basilar membrane 

 Lagena 



Utriculus 

 Sacculus 



Utriculus 

 Sacculus 



Papilla basilaris 

 Basilar membrane 



Lagena 



Cochlea 



Basilar membrane 



Fig. 161. The labyrinths of various vertebrates. A, fish; B, turtle; C, bird; D, mammal. 



From von Frisch.^^ 



canal, and lampreys have two; all higher fishes and higher vertebrates have 

 three. There is little if any differentiation of an auditory mechanism in the 

 cyclostomes. In most fishes the sacculus has a small evagination which is 

 known as the lagena and which is supplied by a separate branch of the eighth 

 cranial nerve. In fishes of the suborder Ostariophysina and in amphibia the 

 lagena is larger, in reptiles, birds, and mammals it has become the cochlear 

 duct. The middle ear containing a single ossicle is first found in amphibians 

 and has developed into the highly efficient structure with three ossicles found 



