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bony labyrinth. In studying this chart-diagram it is in the first 

 place essential to realise that in the membranous labyrinth 

 there are two absolutely distinct structures enclosed one 

 within the other. The inner part known as the endolymph 

 labyrinth is the actual sense organ — the seat of the sensory 

 elements in connexion with the filaments of the otic nerve. It 

 is left white in the diagram. The outer part (the perilymph 

 labyrinth) forms a sheath to the endolymph labyrinth fitting it 

 tightly or loosely in different parts. In the diagram it is 

 represented as partly opened, its cavity being dotted. 



This perilymph sheath is really no part of the sense organ 

 at all but is simply a portion of the mechanism by which 

 vibrations are conducted to the sense organ. The distinction 

 between these two parts cannot be too clearly recognised, for 

 in higher vertebrates and particularly in the cochlea of 

 mammals, parts of the outer sheath are so intimately blended 

 with the enclosed endolymph labyrinth that it is difficult 

 without reference to their past history as revealed by com- 

 parative anatomy to realise that they are not integral parts of 

 a single organ. 



The endolymph labyrinth apart from its perilymph casing 

 can further be conveniently divided for study into two regions 

 physiologically distinct, the one, which forms practically the 

 entire labyrinth in aquatic vertebrates, being an organ for 

 equilibration, the other (peculiar to terrestrial vertebrates) 

 being specialised for audition. In fig. 2 these regions are 

 respectively represented by the parts of the labyrinth known 

 in man as vestibular — that is the semi-circular canals, utricle, 

 saccule and (in lower vertebrates) the lagena — on the one hand ; 

 and the cochlear canal or pars basilaris lagenae (the scala media 

 cochleae of human anatomy), on the other. 



In tracing the evolution of the vestibular or equilibrating 

 part of the labyrinth, it will be unnecessary to consider the 

 perilymph sheath, for this only comes into prominence in 

 terrestrial vertebrates as an accessory to the auditory organ. 



In every endolymph labyrinth, except only those of the 

 Lampreys and Hag-fishes, there are certain constant features 

 subject of course to minor variation but always recognisable. 

 Three semi-circular canals surmount and open into the saccular 

 chambers that form usually the bulk of the labyrinth. Each 

 canal has always at one end a swelling (fig. 2, AMP.) crossed 



