AUDITORY ORGANS. 435 



Accessory structures connected with the organ of hearing in 

 Terrestr^ial Vertebrata. 



In all the Amphibia, Sauropsida and Mammalia, except the 

 Urodela and a few Anura and Reptilia, the first visceral or hyo- 

 mandibular cleft enters into intimate relations with the oro^ans of 

 hearing, and from it and the adjoining parts are formed the tympanic 

 cavity, the Eustachian tube, the tympanic membrane and the meatus 

 auditorius externus. The tympanic membrane serves to receive from 

 the air the sound vibrations, which are communicated to fluids con- 

 tained in the true auditory labyrinth by one ossicle or by a chain of 

 auditory ossicles. 



The addition to the organ of hearing of a tympanic membrane to 

 receive aerial sound vibrations is an interesting case of the adaptation 

 of a structure, originally required for hearing in water, to serve for 

 hearing in air ; and as already pointed out, the similarity of this mem- 

 brane to the tympanic membrane of some Insects is also striking. 



There is much that is obscure with reference to the actual de- 

 velopment of the above parts of the ear, which has moreover only 

 been carefully studied in Birds and Mammals. 



The Eustachian tube and tympanic cavity seem to be derived 

 from the inner part of the first visceral or hyomandibular cleft, the 

 external opening of which becomes soon obliterated. KoUiker holds 

 that the tympanic cavity is simply a dorsally and posteriorly directed 

 outgrowth of the median part of the inner section of this cleft; 

 while Moldenhauer (No. 392) holds, if I understand him rightly, 

 that it is formed as an outgrowth of a cavity called by him the sulcus 

 tubo-tympanicus, derived from the inner aperture of the first visceral 

 cleft together with the groove of the pharynx into which it opens ; 

 and Moldenhauer is of opinion that the greater part of the original 

 cleft atrophies. 



The meatus auditorius externus is formed at the region of a 

 shallow depression where the closure of the first visceral cleft takes 

 place. It is in part formed by the tissue surrounding this depression 

 growing up in the form of a wall, and Moldenhauer believes that this 

 is the whole process. Kolliker states however that the blind end 

 of the meatus becomes actually pushed in towards the tympanic 

 cavity. 



The tympanic membrane is derived from the tissue which sepa- 

 rates the meatus auditorius externus from the tympanic cavity. 

 This tissue is obviously constituted of an hypoblastic epithelium on 

 its inner aspect, an epiblastic epithelium on its outer aspect, and a 

 layer of mesoblast between them, and these three layers give rise to 

 the three layers of which this membrane is formed in the adult. 

 During the greater part of foetal life it is relatively very thick, and 

 presents a structure bearing but little resemblance to that in the 

 adult. 



A proliferation of the connective tissue-cells in the vicinity of 



28—2 



