AUDITORY ORGAN 



293 



of the sensory plate or macula acustica, which are originally con- 

 tinuous, become disconnected from one another later, and from 

 Teleosts onwards are seen as separate maculae acusticse. 



A number of the regions characterised by the possession of 

 this sensory epithelium are not concerned with the sense of 

 hearing: those of the ampulla', for instance, and probably those 

 situated in the utriculus, are to be looked upon as organs of 

 equilibration, serving for the orientation of the individual in space. 

 The fact that the above-mentioned parts of the membranous 

 labyrinth are phylogenetically very ancient structures, and also 

 that the organ in Invertebrates to which the function of hearing 

 was formerly ascribed has now been proved to be concerned mainly 

 with equilibration, indicates that the latter function was the primary 



Fio. 214. LONGITUDINAL SECTION OF AN AMPULLA OF Gomus. (After Hensen.) 



a, base of semicircular canal ; b, point of opening of the ampulla into the alveus 

 communis ; c, epithelium on the wall of the ampulla ; d, auditory hairs ; n, 

 nerve passing into the connective-tissue of the crista. 



one in the vertebrate membranous labyrinth. Later, on the gradual 

 appearance of the cochlea, a division of labour took place, the 

 more ancient part of the labyrinth retaining its primitive function, 

 while the new portion by degrees became differentiated into 

 a specialised auditory organ serving for the analysis of sound 

 Vibrations. 



The higher we pass in the Vertebrate series, the greater share 

 does the mesoderm take in the formation of the auditory organ. 

 At first that is, in Fishes the auditory capsule lies close to the 

 surface, and is thus easily accessible to the waves of sound, which 

 are conducted partly through the operculum (when present), and 

 partly through the gill-cavities or spiracle. In the higher animals 

 the auditory organ gradually sinks further and further inwards 



