AUDITORY ORGAN. 



197 



three ampullae of the canals, in each of which the auditory cells 

 are situated on a ridge (crista acustica) projecting into the 

 lumen ; (2) the utriculus, in which a large " macula acustica " is 

 present; this is continued into the recessus utriculi as well as into 

 the sacculus and the rudiment of the cochlea (the recessus cochleae), 

 which arises from the sacculus. The several portions of the sensory 

 plate or macula acustica, which are originally continuous, become 

 later disconnected from one another, and from the Teleostei onwards 

 are seen as separate maculae acusticse. 1 



FIG. 162. LONGITUDINAL SECTION OF AN AMPULLA OF GOBIUS. (The exact form 

 of the epithelium of the crista is not indicated.) (After Hensen. ) 



11, the nerve passing into the connective-tissue of the crista ; a, base of semicircular 

 canal ; b, point of opening of the ampulla into the alveus communis ; c, the 

 somewhat cylindrical epithelium on the free wall of the ampulla ; d, the 

 auditory hairs. 



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

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

 At first, that is, in Fishes, the ear lies close under the roof of the 

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

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

 through the gill-slits or spiracle : 2 as we pass to the higher animals, 



form and size. The largest and most massive ones are seen in Teleosteans. They 

 either consist of a single mass, or are arranged in groups in different regions of the 

 labyrinth. Nothing certain is known as to their physiological function. 



1 Besides the above-mentioned areas of distribution of the auditory nerve, there 

 is still another independent one : it lies in Fishes on the floor of the utriculus, close 

 to the duetus sacculo-utricularis, to be described present!}', and is called the macula 

 neglecta. It is present from Fishes up to Birds, lying in Amphibia on the inner 

 side of the sacculus, and in Keptiles and Birds in the utriciilus, as in Fishes : in 

 Mammals it undergoes a gradual reduction, and finally becomes obliterated. In all 

 Vertebrates except Mammals, all the auditory nerve-endings are characterised by only 

 two forms of celJs (auditory and supporting cells) : in the Mammalian cochlea the 

 cells of the sensory epithelium possess a peculiar form. 



'-' Howes has shown that in the Skate the structure known as the "fenestra 

 vestibuli cartilaginei " corresponds physiologically to a kind of tympanum. 



