AUDITORY ORGAN 



291 



The whole of this complicated apparatus constitutes the internal 

 ear or membranous labyrinth. It becomes surrounded secondarily 

 by mesodermic tissue, which is at first gelatinous and in close 

 contact with it. A process of absorption then takes place in the 

 innermost layers of the mesoderm, and thus a space is developed 

 which closely repeats the form of the membranous labyrinth, as 

 does also the mesoderm which encloses this space and which later 



FIG. 212. ISOLATED ELEMENTS OF THE MEMBRANOUS LABYRINTH OF VARIOUS 

 VERTEBRATES. (After G. Retzius. ) 



A, from the macula acustica communis of Myxine, ylutinosa ; B, from the macula 

 acustica neglecta of Raia davata ; C, from the crista acustica of an ampulla 

 of Amblystoma ; D, from the crista acustica of the anterior ampulla of 

 Jtana escnlenta. 



f~, thread-like cells ; hz, hair- cells with auditory hairs (h) ; n, nerve. On the 

 left side of D the auditory hair has become broken up into its constituent 

 fibres. 



becomes chondrified, and often also ossified. A membranous and a 

 cartilaginous or bony labyrinth can thus be distinguished, and 

 between them is a cavity (cavum perilymphaticum) tilled with a 

 lymph-like fluid (perilymph), and penetrated by connective tissue 

 and blood-vessels extending from its walls to the membranous 

 labyrinth, The cavity within the latter, which also contains a 

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



In Amphioxus, an auditory organ is wanting. In all Craniates, 

 except Cyclostomes (p. 295), three semicircular canals are present, 



U 2 



