70 COMPARATIVE ANATOMY 



vitreous humour. The sclerotic is lined by a black pig- 

 mented layer called the choroid, which is continued in front 

 into the iris. Inside of this, lining the chamber of the eye, is 

 a delicate transparent membrane, the retina, on which the 

 ramifications of the optic nerve are spread. It is the sensitive 

 portion of the eye, and the cornea, iris, lens, etc., are structures 

 adapted to focus the light upon it. 



The frog's ear is embedded in the periotic capsule, which, as 

 we have seen, is largely cartilaginous, with an anterior ossifica- 

 tion, the pro-otic. The essential organs of hearing are deeply 



~-?-&c. 



'fag. 



Fig. 12. 



The right membranous labyrinth of the frog's ear 

 seen from the inner side. U, utriculus, from which 

 spring ace, the anterior semicircular canal with 

 its ampulla aa : pec, the posterior circular canal 

 with its ampulla amp ; ext.cc, the external 

 horizontal semicircular canal, with its ampulla 

 amp, represented as showing through the trans- 

 parent walls of the utriculus ; S, the sacculus ; de, 

 the ductus endolymphaticus ; lag, lagena ; pbc, 

 pars basilaris cochleae. (Adapted from Retzius.) 



embedded in the cartilage, and only communicate with the 

 surface by an accessory apparatus consisting of the tympanic 

 cavity, the eustachian tube, and columella. 



The tympanic cavity, bounded externally by the tympanic 

 membrane, is seen, after removal of the latter, as a shallow, 

 funnel-shaped cavity, lined by a pigmented mucous membrane, 

 and communicating by the wide, short passage of the eusta- 

 chian tube with the pharynx. The inner and deeper part 

 of the cavity presents a small aperture leading into the inner 

 ear. This aperture, called the fenestra ovalis, is blocked 

 by the enlarged cartilaginous end of a partly bony, partly 

 cartilaginous rod, the columella auris, which traverses the 



