COMPARATIVE ANATOMY 



repeats the form of the membranous labyrinth, as does also the 

 mesoblast which encloses this space and which later becomes 

 chondrified, and often also ossified. A membranous and a bony laby- 

 rinth can thus be distinguished, and between them is a cavity 

 (cavum perilymphaticwri) filled with a lymph-like fluid {perilympK). 

 The cavity within the membranous labyrinth, which also contains a 

 fluid (endolymph), is spoken of as the cavum endolymphaticum. 



Except in Cyclostomes, 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 (Fig. 177). The first 

 and last-named arise from the portion of the utriculus known as 

 the recessus utricuii, and each has a vesicle-like swelling or ampulla 



\ a 



FIG. 178B. LONGITUDINAL SECTION OF AN AMPULLA OF GOBIUS. (The exact form 

 of the epithelium of the crista is not indicated.) After Hensen. 



11, the nerve passing into the connective-tissue of the crista; a, base of semi- 

 circular canal ; b, point of opening of the ampulla into the utriculus ; c, the 

 epithelium on the free wall of the ampulla ; d, the auditory hairs. 



at its origin. The posterior canal also arises with an ampulla from 

 a prolongation of the utriculus (sinus posterior). The other end of 

 the horizontal canal opens by a funnel-shaped enlargement into the 

 utriculus, while that of the anterior and of the posterior canal 

 fuse together to forma common tube, the so-called canal commissure 

 (sinus superior) , which also opens into the utriculus. 



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. These otoliths present the greatest 

 variety both in form and size. The largest and most massive ones 

 are seen in Teleosts. They either consist of a single mass, or are 

 arranged in groups in different regions of the labyrinth. 



