532 THE SENSE OF HEARING. 



tioued, communicates with the vestibule ; another, called fenes- 

 tra rotunda, is separated by a membrane from the cavity of 

 the tympanum ; the third is the orifice of the aquceductus 

 cochleae, a canal leading to the jugular fossa of the petrous 



bone, and corresponding, at 



FlG - 20 - least in obscurity of pur- 



pose and origin, to the 

 aquseductus vestibuli. The 

 spiral canal is divided into 

 two passages or scalse by a 

 partition of bone and mem- 

 brane, the lamina spiralis. 

 The osseous part or zone of 

 this lamina is connected 



View of the osseous cochlea divided through w [fa t ^ e mo diolus; the 



the middle (from Arnold). 5. 1, central mpmhranrms nart with a 



canal of the modiolus; 2, lamina spiralis r ' P art ' W . 8 



ossea ; 3, scala tympani ; 4, scala vestibuli ; 5, mUSCU Jar zone, according 



porous substance of the modiolus near one of to Todd and Bowman, 



the sections of the canalis spiralis modioli. forming its OUter margin, 



is attached to the outer 



wall of the canal. Commencing at the base of the cochlea, 

 between its vestibular and tympanic openings, they form a 

 partition between these apertures ; the two scalse are, therefore, 

 in correspondence with this arrangement, named scala ves- 

 tibuli and scala tympani. At the apex of the cochlea, the 

 lamina spiralis ends in a small hamulus, the inner and concave 

 part of which, being detached from the summit of the modio- 

 lus, leaves a small aperture named helicotrema, by which the 

 two scalse, separated in all the rest of their length, communi- 

 cate. 



Besides the scala vestibuli and scala tympani, there is a third 

 space between them, in the substance of the lamina spiralis, 

 called the scala media, or canalis membranacea, and in this are 

 some peculiar club-shaped little bodies called the rods of Corti, 

 set up on end, with their big extremities upwards, and leaning 

 against each other at the top a section, therefore having the 

 appearance of the gable-end of a house. On their outer part 

 are numerous cells of various shapes. The regularity with 

 which the little rods of Corti are arranged has caused them to 

 be compared to rows of keys in a piano. 



In close relation with these rods and the cells outside them, 

 and probably projecting also by free ends into the little tri- 

 angular canal containing fluid which is between the rods, are 

 filaments of the auditory nerve. 



The membranous labyrinth corresponds generally with the 

 form of the osseous labyrinth, so far as regards the vestibule 



