220 



ANATOMY OF VERTEBRATES. 



163 



The labyrinthic cavity of the right side, magnified 

 two diameters, Human, xcviv. 



them, by sieve-like plates, to the labyrinth; an anterior main one, 

 ib. ?, going to the cochlea, and posterior ones, ib. g, m, supplying 

 the vestibule and semicircular canals. 



The labyrinth is lined by a delicate membrane closing, as it 

 passes, the fenestra tympani, whence it is plainly continued into 

 the cochlea, and completes the spiral septum of that part : con- 

 tinued over the vestibule, the lining membrane is applied to 

 the base of the stapes which closes the ' fenestra vestibuli,' and 

 it lines the semicircular canals. This membrane also extends 

 along two very narrow canals continued from the labyrinth 

 to the exterior of the petrosal, where it passes into the peri- 

 osteum or dura mater of that 

 part. One of the canals com- 

 mences at the vestibule, at 

 /?, fig. 163; the other from 

 the tympanic f scala ' of the 

 cochlea, at v : the serous fluid 

 of the labyrinth passes through 

 these canals to the general 

 arachnoid receptacle of the cere- 

 bral serosity, and they were ac- 

 cordingly termed ' aqueducts,' 

 and distinguished as ' vestibular ' and ' cochlear.' Minute blood- 

 vessels are continued along both canals ; but their constancy and 

 their relation as the intercommunicating medium between the 

 acoustic and cranial serosity indicate a function which justifies 

 the precision with which they have been described by Cotugno. 1 

 The anthropotomical ( aqueducts ' show the last trace of that com- 

 munity, so extensive in fishes (vol. i. fig. 227), in the differentiation 

 of the cranial from the otocranial cavities. 



The mammalian cochlea consists of a spiral tube, fig. 163, d, r, t, 

 usually describing two turns and a half, and narrowing toward 

 the apex, the vaulted roof of which forms the cupola,' fig. 164, c. 

 The internal wall of the cochlear spire and the space it includes 

 form the ( modiolus,' ' columella,' or hollow central pillar, ib. i, 2, 

 which, from the wider sweep taken by the first turn, is broadest 

 below. Here enters the trunk, ib. , a, of the cochlear division 

 of the acoustic nerve, and the foramina by which its fibrils pene- 

 trate the spiral canal extend along a part of the modiolus called 

 ' tractus spiralis foraminulentus.' The tube of the cochlea is 

 divided into two passages or ' scalar ' by a delicate plate of bone, 

 fig. 163, q, q, attached to the inner or modiolar wall of the turns, 

 and projecting freely into their cavity toward the outer wall : the 



1 xcv. 



