AUDITORY ORGAN 295 



takes on a close relation to the auditory apparatus, and gives rise 

 to a spacious chamber, the tympanic cavity or middle ear, communi- 

 cating with the pharynx by the Eustachian tube, and being closed 

 externally by a vibratory tympanic membrane, between which and 

 the auditory capsule a sound-conducting apparatus (columella or 

 auditory ossicles, pp. 98, 133) extends. In higher forms still, the 

 tympanic membrane is situated more deeply at the base of an 

 external auditory meatus or passage, to which in Reptiles the first 

 indications of a muscular fold of the integument are added. Only 

 in Mammals, however, does the meatus play an important part, and 

 in them a typical pinna or auricula is developed, which, with the 

 meatus, constitutes what is known as the external ear. 



Cyclostomes. In Petromyzon there are only two (vertical) 

 semicircular canals, which unite together in a common section, the 

 so-called commissure ; in Myxine only one canal is present, but 

 this, as it possesses two ampullae, probably represents the two 

 united with one another (Fig. 216, A). 



Fishes. The auditory organ of all true Fishes (Fig. 216, A x -c) 

 follows the general plan given above, and the same may be said for 

 all the higher Vertebrates. Almost without exception we meet 

 with a division into a pars superior represented by the utriculus 

 and semicircular canals, which remains essentially much in the 

 condition already described, and a pars inferior constituted by 

 the sacculus and cochlea, which gradually becomes more differ- 

 entiated, and attains a higher and higher degree of development 

 and functional perfection. In Fishes, the lagena or cochlea consists 

 simply of a small knob-like appendage of the sacculus, which opens 

 freely into the main cavity of the latter by means of the sacculo- 

 cochlear canal : it is absent in Chimaera. The utriculus and sacculus 

 also communicate with one another by the sacculo-utricular canal. 1 

 In Elasmobranchs the ductus endolymphaticus opens to the 

 exterior dorsally, and is thus in free communication with the sea- 

 water.' 2 In certain Teleosts, in addition to a large otolith situated 

 in the sacculus and smaller ones in the lagena and recessus utriculi 

 respectively, calcareous masses may be present in outgrowths from 

 the sacculus. 



In Chimaeroids, Ganoids, Teleosts, and Dipnoans the auditory 

 capsules are not completely surrounded by cartilage or bone, the 

 perilymphatic and cranial cavities only being separated by a fibrous 

 partition. As compared with the practically independent carti- 



1 In Plagiostomes the utriculus and sacculus are not divided off from one 

 another, and the anterior and posterior canals do not unite in a sinus utriculi. 

 In Rays the canals are almost completely circular. 



2 The endolymphatic duct corresponds to the stalk of the auditory vesicle 

 connecting it with its superficial point of origin ; in Vertebrates other than 

 Elasmobranchs (except in Teleosts, in which it is wanting) it becomes separated 

 very early from the outer ectoderm. 



