536 



COMPARATIVE ANATOMY. 



from the lower divisions upwards. In Fishes there is an indication 

 of it in the form of a diverticulum, generally a small one, of the 

 sacculus. In the Selachii it contains a number of small otoliths; 

 in the Teleostei one larger one (asteriscus). In the Amphibia this 

 diverticulum of the sacculus is moi'e independent, but it is still 

 connected as before, and is still directed backwards. 



This portion, which carries the end of a branch of the auditory 

 nerve, is still further differentiated in the Reptilia and Aves, where 

 the diverticulum, which forms it (Fig. 300, D E c), is a short 

 conical piece, which is directed downwards from the median wall of 

 the labyrinth, and converges towards its fellow of the opposite side. 

 Its end is somewhat bent, and it forms the " lagena." Among 

 Mammals this stage of the organ is seen in the Monotremata only j 

 in the rest this stage is not the permanent one, for the organ is 

 converted into a spirally-coiled canal. At first it is only formed by 

 a prolongation of the sacculus, but special differentiations appear in 

 it, and this cochlear canal, which is formed from the sacculus, is 

 permanently connected with it by a narrower portion only (canalis 

 rouniens, Fig. 302). The organ, which thus becomes more indepen- 

 dent, is sur- 

 E rounded on 



two sides of 

 its course by 

 lymphatic ca- 

 vities, which 

 accompany it 

 in its coils, 

 and pass into 

 one another 

 at the apex 

 of the cochlea. 

 One cavity 

 is connected 

 with the os- 

 seous vesti- 

 bule, the other 

 is shut off 

 from it at 

 its commence- 

 ment, and is 

 only con- 



nected with 



the cavity of the vestibule indirectly ; that is by the communication 

 at the apex of the cochlea. Three cavities, therefore, can be dis- 

 tinguished in the Mammalian cochlea; but ono only, the ductus 

 cochlearis, is connected with the vestibular labyrinth. The other 

 two form the scalre — the sc. vestibuli and sc. tympani. The two 

 scalse occupy the periphery of the coils of the ductus cochlearis, 

 at the base of which the end-organs of the cochlear nerves (organ 



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Fig. 302. Diagrams in explanation of the labyrinth. / Fish. 

 // Bird. /// Mammal. 17 Utriculus. S Sacculus. US Utricuhis 

 and Sacculus. Cr Canalis reunions. R Eocessus labyrinthi. 

 UC Commencement of the cochlea. C Cochlea. L Lagena. 

 K Ca?cal sac at apex. C Caccal sac of the vestibulum of the 

 cochlear canal (after Waldeyer). 



