82 MANUAL OF ELEMENTARY ZOOLOGY 



in planes at right angles to one another, one of them 

 being horizontal, another longitudinal- vertical (the posterior 

 vertical), and another transverse vertical (the anterior 

 vertical). One of the ends of each of them is enlarged to 

 form a small, rounded ampulla. From the sacculus arise 

 three small dilatations which represent the cochlea of higher 

 animals. Between the auditory capsules and the membrane 

 of the ear-drum or tympanic membrane on the side of the 

 head lies the cavity of the ear-drum or tympanic cavity, which, 

 as we have seen, is crossed by the columella from the 

 fenestra ovalis to the tympanic membrane and communi- 

 cates with the pharynx by the Eustachian tube. This region 

 is called the middle ear, the labyrinth being the inner ear. 

 There is no outer ear in the frog. 



The semicircular canals are not organs of hearing, but 

 enable the animal to keep its balance by 

 the*Eara! ° f 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. When they are diseased or injured 

 giddiness is caused. 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. 



The organs of smell are a pair of irregular chambers, 

 enclosed by the nasal capsules, separated by 

 organ!? tne 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, 



