EARS 391 



the Eustachian tube. This region is called the middle ear, the 

 labyrinth being the inner ear. There is no outer ear in the frog. 



FUNCTIONS OF THE EARS 



The semicircular canals and utriculus are not organs of hearing, 

 but enable the animal to keep its balance by judging the position 

 of its head. Placed as they are in three planes of space, the fluid 

 in them is set in movement by any change in position, and 

 the differences in pressure on their walls which are thus brought 

 about start impulses which the auditory nerve conveys to the 

 brain. The true organ of hearing is the sacculus. The vibrations 

 which constitute sound set the tympanic membrane in motion, 

 and its movements are transferred by the columella to the 

 membrane of the fenestra ovalis and thence through the perilymph 

 and the wall of the membranous labyrinth to the endolymph, 

 where they stimulate the endings of the auditory nerve in the 

 dilatations of the saccule. Frogs appear to be able to respond to 

 sounds up to a frequency of about 500 cycles a second. 



OLFACTORY ORGANS 



The organs of smell are a pair of irregular chambers, enclosed 

 by the nasal capsules, separated by the nasal septum, and 

 communicating with the exterior by the nostrils and with the 

 mouth by the internal nares. The lining of each is connected with 

 the olfactory nerve of its side. Air is drawn through the chambers 

 in the process of breathing, and the odorous particles it contains 

 affect certain cells of the lining which are connected with fibres 

 of the nerve. Anurans, however, seem to be relatively insensible 

 to smell. 



