1096 



SPECIAL SENSE ORGANS 



The cochlear duct (membranous cochlea or scala media) begins within the 

 cochlear recess of the vestibule in a blind pouch, the vestibular caecum, and 

 traversing the spiral canal of the cochlea, ends just beyond the hamulus of the 

 lamina spirahs in a second blind pouch, the cupular caecum. Close to the ves- 

 tibular caecum it is joined to the saccule by the ductus reuniens. It is lined 

 throughout bj^ epithelium and is somewhat triangular in cross-section. Its 

 floor is formed by thickened periosteum over part of the osseous lamina spiralis 

 and by a fibrous membrane, the lamina basilaris, which stretches from the free 

 border of the lamina spiralis to a thickening of the periosteum, the spiral ligament 

 of the cochlea, on the peripheral wall. 



The epithelium of this floor is greatly modified, forming the spiral organ (organ of Corti) 

 in which the fibres of the cochlear nerve terminate. The peripheral wall is formed by the 

 thickened periosteum upon the peripheral wall of the cochlear canal, while the third wall is 



Figs. 843 and 844. — Sections SnowaNG Early Stages in the Development of the Otic 



Vesicle. 









I 



fomed by a thin vestibular membrane (membrane ot Reissner) which passes from the periphera 

 wall to the osseous lamina spiralis near its free margin, forming with the lamina spiralis an angle 

 Ff 45 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 scala tympani unites with the scala vestibuli 

 at the helicotrema, and from the scala tympani a minute canal, the perilymphatic duct, passes 

 through the cochlear canaliculus and connects with the subarachnoid space. A thin fibrous 

 layer, the secondary tympanic membrane, closes the cochlear fenestra (fenestra rotunda) 

 and thus separates the scala tympani from the tympanic cavity, and the vestibular perilym- 

 phatic space (scala vestibuli) 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 , fig. 514), a branch of the basilar artery, 

 accompanies the cochlear and vestibular nerve. It supplies the vestibule, semicircular canals, 

 and cochlea, and their membranous contents. The blood is returned by the internal auditory 

 vein into the inferior petrosal sinus, and by small veins which pass through the cochlear and 

 vestibular acjueducts to the inferior and superior petrosal sinuses. The acoustic nerve (p. 949, 

 figs. 841 and 842) consists of a vestibular and a cochlear division. The membranous ampulla; of 

 the semicircular ducts and the acoustic macula? of the utricle and saccule are supplied by 

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

 the cochlear nerve. 



Development of the Ear 



The external and middle ears liavc a common origin quite distinct from that which gives 

 rise to the internal oars, and arc to be regarded as portions of the branchial arch apparatus 

 secondarily adapted to auditory purposes. 



The sensory epithelium lining the internal car is derived from the otic vesicle, a structure 

 formed from th(; surface epithelium of the head, while the membrane and bones surrounding 

 it are formed from the mesoderm which surrounds the vesicle. 



