334 



THE BIOLOGY OF THE FROG 



CHAP. 



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

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

 duces the reverse movement. The eye is pulled into the 

 orbit by means of the strong retractor biilbi 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 



Fig. 93. — Right membranous labyrinth of the frog from the inner side. 

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

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

 cus, utriculo-saccular canal; rf^, ductus endolymphaticus ; /, lagcna; tnu, 

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

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

 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; 55, j/, sinus utriculi superior 

 and posterior ; a, 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 



