530 TEXTBOOK OF ZOOLOGY 



Sharpness of vision is dependent on both the proper focusing of 

 the lens and the proper amount of light reaching the retina. When 

 the light is too strong, the pupil of the iris contracts and cuts down 

 the volume. The eye of the frog has little if any accommodation or 

 focusing of the lens. It therefore has very imperfect vision. 



The ear of the bullfrog is covered externally by a membrane, the 

 tympanum. A Eustachian tube runs between the middle ear and 

 mouth cavity. The tympanum has attached to it a bony rod, the 

 columella, the other end of which is joined to a portion of the inner 

 ear. This rod transmits sound vibrations from the tympanum to the 

 inner ear. 



The inner ear lies in a cavity of the skull known as the auditory 

 capsule. The structures of the inner ear compose a membranous 

 labyrinth which is surrounded by a lymphlike fluid, the perilymph. 

 The labyrinth is formed of a dorsal utriculus concerned with equilib- 

 rium and a ventral sacculus functioning as an auditory organ. The 

 utriculus is connected with three semicircular canals which are placed 

 in planes almost at right angles to one another. Two are vertical 

 canals, and the third, on the outer side of the utriculus, is hori- 

 zontal. The sacculus is irregular, pouchlike, and filled with a fluid, 

 the endolymph. It also contains the nerve endings which receive the 

 stimuli and convey them to the auditory nerve. 



Sound progresses in the following fashion. The tympanic mem- 

 brane vibrates to sound waves, and these are transported by the 

 columella to the inner ear. These vibrations are taken up by the 

 endolymph of the sacculus and are received by the nerve endings 

 which lead to the auditory nerves. These nerves convey the im- 

 pulse to the brain, subsequently giving rise to auditory sensations. 



In a similar manner, movements of the endolymph in the utricu- 

 lus affect sensory cells and cause a reaction associated with a sense 

 of position or equilibration. 



Sound and hearing play an important role in the life of frogs, 

 the calls of the males serving to attract the females and others to 

 the ponds during the breeding season. They are of prime importance 

 in the daily life of the terrestrial toad, who is on the alert when 

 an insect has announced its location by a sound. 



