196 COMPARATIVE ANATOMY. 



get a membranous and a bony labyrinth, and between them 

 a cavity (cavum perilymphaticum) filled with a lymph-like fluid 

 (peri lymph). The cavity within the membranous labyrinth, 

 which also contains a fluid (endolymph), is spoken of as the 

 cavum endolymphaticum. 



With the exception of the Cyclostomi, three semicircular canals 

 are always present, and these lie in planes at right angles to one 

 another. They are distinguished as the anterior vertical, the 

 posterior vertical, and the horizontal (external) canals. 

 The first and last-named (Fig. 161, S.B., H.B^) arise from the 

 portion of the utriculus known as recessus utriculi (Re.ut^), and 

 each has a vesicle-like swelling or ampulla (S.A., If A.} at its 



Co. 

 S.B 



// J!;,r5\ 



F.B. 



JI.B: 



FIG. 161. SEMIDIAGRAMMATIC FIGURE OF THE AUDITORY ORGAN OF A 

 TELEOSTEAN. (Modified from a figure of that of Murcena anguilla by Hasse.) 



Ut., utriculus ; Re.ut., recessus utriculi ; V.B., connecting-tube of the posterior ampulla 

 (F.A.); **, wide connecting-duct between the pars superior and pars inferior ; 

 8, sacculus ; C, cochlea; S.B., F.B,, H.B., anterior and posterior vertical, and 

 horizontal canals ; Co. , canal commissure, with its apex ; S.A., H.A., F.A., 

 ampullae of the anterior, horizontal, and posterior canals ; D.e, ductus endo- 

 lymphaticus, which arises from the point where the two tubes of the pars 

 superior of the labyrinth and the opening of the horizontal canal ( x ) join one 

 another. 



origin. The posterior canal (F.B.) also arises with an ampulla 

 (F.A.) from a prolongation of the utriculus, described in Fig. 161 

 as the " connecting-tube " ( V.M.). 



The other end of the horizontal canal opens by a funnel-shaped 

 enlargement (Fig. 161, x) into the utriculus, while those of the 

 posterior and anterior canals fuse together to form a common 

 tube, the so-called canal commissure (sinus superior) (Co.), 

 which also opens into the utriculus. 



The distribution of the branches of the auditory nerve and the 

 position of the sensory epithelium in the following parts of 

 the membranous labyrinth must now be considered : 1 (1) the 



1 Concretions composed mainly of carbonate of lime are present in the regions 

 of the various nerve end-plates of the auditory organ in all Vertebrates, as well as 

 iii many Invertebrates. These "otoliths" present the greatest variety both in 



