TIIK l.\THh'.\AL KM; 1061 



(organ of Corti), in which the fibres of the cochlear nerve terminate. The periph- 

 eral wall is formeil by I he 1 liiekened periosteum upon t he peripheral wall of the coch- 

 lear canal, while the third wall is formed by a thin vestibular membrane (mem- 

 brane of Reissner) which pas-.es from the peripheral wall to the o-seous lamina 

 spiralis near its free margin, forming with the lamina spiralis an angle of \~> degrees. 

 The cochlear duct and the osseous spiral lamina divide the cochlear spiral canal into 

 two parts, one next to the basilar membrane, the scala tympani, and one next to 

 the vestibular membrane, the scala vestibuli. The ,-cala tympani unites with the 

 scala vestibuli at the helicot renia. and from (ho .scala tympani a minute canal, the 

 perilymphatic duct, passes through the cochlear canaliculus and connects with the 

 suliaraclmoid space. A thili fibrous layer, the secondary tympanic membrane, 

 closes the cochlear fenestra (fenestra rotunda) and thus separate- the scala tympani 

 from the tympanic cavity, and the vestibular perilymphatic space (scala vestilmli) 

 is separated from the tympanic cavity by the base of the stapes in the vestibular 

 fenestra (fenestra ovalis). 



Vessels and nerves. The internal auditory artery (p. 546, fig. 477), a branch of the basilar, 

 accompanies the acoustic nerve. It supplies the vestibule, semieireular canals, and cochlea, and 

 their membranous contents. The blood is returned by the internal auditory vein into the in- 

 ferior petrosal sinus, and by small veins which pass through the cochlear and vestilmlar aqueducts 

 to the inferior and superior petrosal sinuses. The acoustic nerve (p. <7i), tips. ~>W and 7">M 

 consists of a vestibular and a cochlear division. The membranous ampulUe of the semicircular 

 ducts and the acoustic macula; of the utricle and saceule are supplied by the vestibular nerve. 

 The spiral organ (organ of Corti) in the cochlear duct is supplied by the cochlear nerve. 



The development of the internal ear. The internal ear is the e>-ential organ of hearing, 

 and appears at an early stage in the embryo as a depression of the ectoderm at the side of the head, 

 above the dorsal end of the second branchial arch. The depression is lined by a thickened 

 epithelium, and, sinking more deeply into the subjacent mesodermic tissue, is eventually con-* 

 stricted from the ectoderm to form a rounded sac, the otocyst, imbedded in the mesodermic 

 tissue which \vill become the petrous portion of the temporal bone. Krorn this simple ectodermic 

 s-ic the entire membranous internal ear is formed, the so-called osseous internal ear being merely 

 the bony walls of the cavity in which the otocyst lies. 



