528 THE TYMPANIC CAVITY. 



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 membrane to the tympanic membrane of 

 some Insects is also striking. 



There is much that is obscure with reference to the actual 

 development 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. Kolli- 

 ker 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 

 separates 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 

 the tympanic cavity causes in Mammalia the complete or nearly 

 complete obliteration of the cavity during foetal life. 



The tympanic cavity is bounded on its inner aspect by the 

 osseous investment of the internal ear, but at one point, known 



