390 



THE FROG. PHYLUM CHORDATA 



utr. 



a.v.8. 



p. V. 8. 



amp 



'"■amp. 



•n. 



sac. 



**• hs. chl. 



Pig. 305.— The labyrinth of the right 

 ear of the frog, seen from the outer 

 side. — Partly after Marshall. 



avs Anterior vertical semicircular canal ; 

 amp., ampullae ; chl., small dilatations of 

 the sacculus which represent the cochlea 

 of higher animals ; h.s., horizontal semi- 

 circular canal ; n., branches of the auditory 

 nerve to supply the ampullae ; p.v.s., postenor 

 vertical semicircular canal; sac., sacculus; 

 utr., utriculus. 



EARS 



The essential part of the ear is the membranous labyrinth 

 (Fig. 305) which Hes in the cavity of the auditory capsule. This 



cavity contains a fluid known 

 as perilymph, and the mem- 

 branous labyrinth contains a 

 fluid known as endolymph. 

 The labyrinth consists of the 

 vestibule and the semicircular 

 canals. The vestibule has an 

 upper, larger division, the 

 utriculus, and a lower, smaller 

 sacculus. From the former arise 

 the three semicircular canals, 

 which are arched tubes opening 

 into the utriculus at both ends. 

 They are placed 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 arises an off- 

 shoot known as the lagena which 

 has three small dilatations ; this 

 represents the cochlea of higher 

 animals. On the median side of 

 the sacculus there starts a tube, 

 the ductus endolymphaticus, 

 which enters the cranial cavity 

 and there expands into a thin- 

 walled sac. Between the auditory 

 capsules and the membrane of the 

 ear-drum or tympanic membrane 

 on the side of the head Hes the 

 cavity of the ear-drum or tym- 

 panic cavity, which, as we have 

 seen, is crossed by the columella from the fenestra ovahs to the 

 tympanic membrane and communicates with the pharynx by 



col. 



,periL 



I 



! .sk. 



Eu. 



— lab. 



Fig. 306. — A diagram of the ear of 

 the frog. 



col., Columella ;/.o., fenestra ovalis ; £M.,Eusta- 

 chian tube ; lab., part of the membranous 

 labyrinth, containing endolymph ; m., 

 mouth ; md., mandible ; peril., perilymph ; 

 sk., skull ; tytn., tympanic membrane. 



