AUDITORY ORGANS 



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utriculus, and hence approximately semicircular in outline (fig. 206). 

 In one end of each of these semicircular canals there is a patch 

 of sensory epithelium and the wall expands around this into an 

 ampulla, the ampullae of the anterior and external canals being side 

 by side, that of the posterior canal at its lower end. 



In the lower ichthyopsida there is Uttle differentiation in the sac- 

 culus, but in the higher a pocket, the lagena, is given off from its 

 posterior side, a portion of the sensory epithelium extending into it. 



Fig. 206. — Diagram of the membranous labyrinth of a vertebrate, the sensory areas 

 dotted, ac, anterior semicircular canal; ap, ampullae; ca, cristae acusticae in the ampullae; 

 de, ductus endolymphaticus; he, horizontal (external) canal; /, lagena; ml, mn, ms, mu, 

 macula (of lagena, neglecta, sacculi and utriculi); pc, posterior semicircular canal; s, 

 sacculus; se, saccus endolymphaticus; sue, sacculo-utricular canal; u, utriculus. 



With increasing powers of hearing the lagena becomes greatly elon- 

 gate, until in the mammals it acquires a peculiar development and is 

 known as the scala media, the structure and relations of which are 

 described below. 



In the cyclostomes utriculus and sacculus are not diflferentiated. In the 

 myxinoids there is but a single semicircular canal, with, however, an ampulla 

 at either end. In the lampreys there are two canals, both in the vertical plane, 

 and each with an ampulla at its lower end. 



These parts of the internal ear form the membranous labyrinth 



