ANATOMY OF THE FROG 71 



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 theutriculus ; (2) a lower and smaller divi- 

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

 the sacculus. I'he 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 endoljrmphaticus. 



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 open- 

 ing by its upper end into the utriculus, in common with the 

 anterior canal, and at its lower end it 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 essentially 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 

 sac is divided up and complicated by the projection into it of 

 cartilaginous offsets of the walls of the nasal chambers, and 

 three main subdivisions or sinuses have been recognised in 

 each sac — a dorsal, a ventral, and a lateral sinus. These 

 sinuses are lined by an epithelial membrane which in the 



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