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distinct regions, an upper utricle and a lower saccule. 

 An upgrowth from the saccule forms the endolymphatic duct ; 

 it opens to the exterior in some Elasmobranchs, and in 

 Chimaera, but in higher Craniates it is a blind diverticulum. 

 In Amphibians (e.gr., Frog) this duct becomes spread out 

 and forms a long extension above the spinal cord with 

 outgrowths along the spinal nerves ; the ends of these 

 outgrowths expand round the spinal ganglia and contain 

 the conspicuous calcareous bodies. 



Three bulgings (not synchronous) of the utricle become 

 the semicircular canals/ Each bulging flattens, and by 

 ingrowth of the wall at each side a hollow arched tube 

 (the canal) is formed, opening into the utricle at both ends ; 

 one of the ends is a dilated ampulla, inside which are cells 

 with long sense-hairs. " These canals, arranged in planes 

 at right angles, have for their function the analysis of any 

 rotatory movement into its components in these three 

 planes." Two of the canals (the anterior and the posterior) 

 are vertical, the third is horizontal. 



From the saccule or vestibule a long curved pouch grows 

 out ; this is the auditory organ, the lagena in Reptiles and 

 Birds, greatly enlarged and spirally coiled in Mammals 

 and known as the cochlea. The auditory sense-cells in 

 the cochlea form the complex organ of Corti. 



The entire otocyst or membranous labyrinth is filled with 

 fluid, endolymph, which (except in Mammals) often contains 

 limy otoliths or " ear-stones " associated with the sensory 

 cells, which are connected with branches of the 8th or 

 auditory nerve. Between the labyrinth and the auditory 

 capsule, which are similar in conformation, there is a 

 buffer fluid, the perilymph. 



The foregoing description is strictly that of the internal 

 ear. There is also (in Amphibians and higher Craniates) 

 a middle ear, tympanum or drum ; its cavity opens through 

 the Eustachian tube into the back of the mouth, and is 

 closed externally by the tympanic membrane, which may 

 be at the exterior (e.g., Frog), or at the inner end of a 

 passage (external auditory meatus) the external opening of 

 which is within a pinna or ear-flap projecting from the 

 head. 



Within the tympanic cavity are the ossicles (malleus, 

 incus, and stapes). The handle of the malleus or hammer 

 is attached to the tympanic membrane, and the stapes 

 is fixed to the fenestra ovalis (a membranous window of 

 the vestibule) ; and thus vibrations are transmitted to the 

 labyrinthine fluid. In Amphibians, Reptiles and Birds 

 there is only one ossicle, the columella. 



