334 THE BIOLOGY OF THE FROG CHAP. 



which rotates the eye around its optic axis so that its upper 

 margin turns forward, and an inferior oblique, which pro- 

 duces the reverse movement. The eye is pulled into the 

 orbit by means of the strong retractor btilbi muscle, which 

 arises from the angle between the lateral and median por- 

 tions of the parasphenoid bone and is broadly inserted upon 

 the posterior and median sides of the eyeball. The eyes 



C(t 



ms 



FlG. 93. Right membranous labyrinth of the frog from the inner side. 

 aa, ae, ap, anterior, external, and posterior ampulla; au, aperture of 

 utriculus ; ca, ce, cp, anterior, external, and posterior semicircular canals ; 

 cus, utriculo-saccular canal; de, ductus endolymphaticus ; /, lagcna ; mu, 

 ms, mn, macula acustica of the recesstis utriculi, sacculus, and pars 

 neglecta respectively; pb, pars basilaris ; pl,ppb, papilla acustica lagenae 

 and basilaris ; raa, rap, rs, rn, rl, rb, branches of auditory nerve to 

 anterior ampulla, posterior ampulla, sacculus, macula neglecta, lagena, 

 and pars basilaris respectively; s, sacculus; ss,sp, sinus utriculi superior 

 and posterior; u, utriculus. (After G. Retzius.) 



are pushed outward by the contraction of the levator bulbi, 

 a broad sheet of muscle running obliquely across the ventral 

 side of the orbit. 



The Ear. - - In the ear of the frog there are two sets of 

 organs which are fundamentally different in origin and in 

 function. The one constitutes the inner ear which forms 



