ANATOMY OF THE FROG 71 



tympanic cavity and is fixed by its outer end in the tympanic 

 membrane. The tympanic cavity and eustachian tube 

 together constitute the middle ear, and are derived from the 

 most anterior of the gill-clefts of the tadpole. The inner 

 ear consists of a membranous sac lying in a corresponding 

 cavity of the cartilage of the auditory capsule. Between the 

 cartilage and the sac is a fluid called perilymph. The sac 

 also has fluid contents called endolymph. A constriction 

 partly divides the sac into two portions: (i) an upper and 

 larger division, called the utriculus; (2) a lower and smaller divi- 

 sion, with three small dilatations on its posterior face, called 

 the sacculus. The last named also gives off from its inner 

 and upper border a tubular offset ending in a thin-walled 

 dilatation. This is called the ductus endolymphaticus. 



The utriculus has more complex relations. Three semi- 

 circular canals, an anterior, a posterior, and an external, open 

 into it. Of these the anterior semi-circular canal lies in the 

 median or sagittal plane of the head ; at its anterior end, just 

 where it joins the utriculus, it is dilated to form an ampulla, 

 and its posterior end joins the posterior semi-circular canal 

 which opens, in common with it, into the utriculus. The 

 posterior semi-circular canal lies in the transverse plane ; its 

 upper end opens into the utriculus, in common with the 

 anterior canal, and its lower end dilates into an ampulla be- 

 fore opening into the utriculus. The external canal lies in the 

 horizontal plane, and has an ampulla at its anterior end. Thus 

 the three canals lie at right angles to one another in the three 

 dimensions of space ; they open into the utriculus at both 

 ends, and each has an ampulla at one end. The auditory 

 nerve, passing through an aperture in the inner wall of the 

 auditory capsule, divides into branches, which are distributed to 

 the utricle, the saccule, and the ampullae. In the interior of 

 the membranous labyrinth are the elements sensitive to sound- 

 waves, in the form of modifications of the lining epithelium 

 bearing stiff sensory hairs. There are also peculiar calcareous 

 concretions, called otoliths, especially abundant in the ductus 

 endolymphaticus. 



The olfactory organs of the frog consist of a pair of 

 sacs separated from one another by a median cartilaginous 

 septum, and opening to the exterior by the anterior nares, into 

 the buccal cavity by the posterior nares. The cavity of each 



