ORGAN OF HEARING. 



535 



Fig. 235. 



. peri- 

 nerve. 



A diagram from Breschet, intended, according to him, to give an 

 exact idea of the disposition of the helicotrema. The walla 

 of the vestibular scala are supposed to be removed. 



a a a. represent the osseous and middle zone of the spiral 

 lamina ; its termination in the hamulus or hook is seen ; 

 b b b b. this darker and narrower stripe represents the mem- 

 braneous zone of the spiral septum ; towards the summit of 

 the cochlea it becomes a little broader, and at its termination 

 constitutes by itself alone the septum between the two scalse 

 at their termination ; c. the commencement of the tympanic 

 scala ; d. the external or great margin ; e. the internal mar- 

 gin of the turns of the cochlea ; the two margins d and e 

 meet at o; fff. the vacant space corresponding to the axis ; 

 it terminates at o, which corresponds to the summit of the 

 axis ; x. helicotrema or hole which establishes a communica- 

 tion between the two scalae. 



The section of the peripheral margin of the 

 membraneous zone presents a triangular sur- 

 face, the base of which is inserted on the osse- 

 ous wall of the cochlea. This swollen margin 

 of the membraneous zone is, according to 

 Breschet, evidently continuous, at the origin 

 of the spiral lamina in the base of the cochlea, 

 with the osseous zone, a circumstance which 

 is particularly to be remarked in very young 

 fetuses, where all these parts are still cartila- 

 ginous. This thickened margin of the mem- 

 braneous zone Breschet therefore considers as 

 analogous to the tympanic cartilage of the 

 bird's cochlea, having exactly the same rela- 

 tions and uses. 



Fig. 236. 



pared to the tympanic cartilage of birds ; 

 /. scala vestibuli , g. scala tympani ; h. 

 periosteum lining the vestibular scala, 

 more vascular than fibrous ; f 

 osteum of the tympanic scala ; j. 



In vascularity and richness in 

 nerves, the spiral lamina bears a 

 great resemblance to the iris. Like 

 it, also, it is the partition between 

 two chambers, containing an aque- 

 ous humour, and communicating, 

 like the aqueous chambers of the eye, 

 by a single orifice. 



The two scalae of the cochlea have 

 not the same length nor the same 

 diameter. Toward the base of the 

 cochlea the tympanic scala exceeds 

 somewhat the vestibular; its diameter 

 is, at the same time, also a little more . 

 considerable, as far as towards the 

 middle of the first turn of the spire. 

 The two scalse have then the same 

 diameter, and preserve the equality 

 to the commencement of the last turn. 

 There the tympanic scala contracts, 

 and in particular flattens considerably, 

 and is at last confounded, through the 

 helicotrema, with the vestibular scala, 

 which still continues for two-thirds of 

 a turn, and then ends in a cul-de-sac. 

 This is also to be noted in regard to 

 the vestibular scala of the bird's coch- 

 lea, which indeed is very large, and 

 proceeds considerably beyond the tym- 

 panic scala. It ends in a large cul-de- 

 sac called lagena. 



Fig. 237. 



Diagram of a transverse section of the two scales of 

 the cochlea (from Bresrhet). 



a. a. osseous wall ; b. osseous zone compared to 

 the vestibular cartilage of the cochlea of birds ; e. 

 middle zone compared to the auditive lamellae; 

 d. membraneous zone ; e. cartilaginiform swelling of 

 the external margin of the membraneous zone, com- 



i 



A section of the cochlea parallel to tJie direction of 

 its axis, in order to show the disposition of the vohnle 

 of its parts. Magnified. (From Breschet.) 



a. a. a. trunk of the cochlear nerve ; b. b. fila- 

 ments of this nerve in the osseous zone ; c. c. c. c. 

 nervous anastomoses in the middle zone ; d. d. d. d. 

 membraneous zone ; e. e. e. e. swelling of the exter- 

 nal margin of the membraneous zone ; 1,1, axis 

 of the cochlea; 2, infundibulum ; 3, 3, 3, 3, exter- 

 nal osseous wall of the cochlea ; 4, 4, 4, 4, osseou* 

 lamina separating the turns of the spire of the 

 cavity of the cochlea ; 5, 5, 5, 5, tympanic lamella 

 of the osseous zone of the spiral lamina ; 6. vesti- 

 bular lamella; 7, hamulus or hook, which ter- 

 minates the osseous zone ; 8, helicotrema, with a. 

 bristle introduced into it. 



