ix. ] THE NER VO US S YS TEM. 



393 



The closed sac sends out four processes, three of which 

 become the three semicircular canals, while the fourth de- 

 taches itself, coils round, and becomes the cochlea. In the 

 meantime tissue on the inner side of this "labyrinth" changes 

 into the true auditory nerve, and thus places that structure 

 in direct communication with the brain. 



This merely membranous labyrinth is afterwards enclosed 

 in dense bone as before mentioned in describing the petrous 

 part of the temporal bone the bony investment leaving two 

 apertures (termed respectively fti&fenestra ovalis and the 

 fcnestra rotunda) besides those minute foramina (at the 

 bottom of the meatus auditorius internus) through which 

 the filaments of the auditory nerve enter. 



The visceral cleft meanwhile becomes differentiated into 

 (i) an outermost part (the meatus auditorius externus) in- 

 vested beneath by the tympanic bone, and closed within by 

 the tympanic membrane or drum of the ear ; (2), a median 

 part (the tympanic cavity) bounded externally by the drum of 

 the ear, and traversed by the auditory ossicles, the malleus, 

 incus, and stapes j (3), an innermost part (the Eustachian 

 tube) a straight, simple canal opening internally at the 

 side of the roof of the pharynx by a rather wide aperture. 



The upper ends of the mandibular and hyoidean arches 

 become differentiated into the malleus and the incus respec- 

 tively, the stapes being, as it were, an outgrowth going from 

 the fenestra ovalis to the upper part of the hyoidean arch. 



From the posterior margin of the hyoidean visceral arch a 

 membrane grows out which becomes the external ear, the 

 concha or pinna, \vhich presents various parts, amongst the 

 smaller yet more noteworthy of which are the soft depending 

 portion, or lobule, and the little conical prominence (called 

 the tragus) which projects backwards over the external 

 opening of the ear. 



Such being the condition and development of the ear in 

 man, it remains to estimate its peculiarity and the nature of 

 some of its component parts by a brief survey of the more 

 remarkable conditions which the organ presents in other 

 Vertebrates. 



In that he possesses an internal ear, he agrees with all 

 Vertebrates except the anomalous Lancelet, nor (with this 

 exception) does the organ of hearing ever become atrophied 

 in the way which we have seen such atrophy to occur occa- 

 sionally with the organ of sight. 



A labyrinth composed of three semicircular canals is also 



