476 



SCIENCE. 



[Vol. I., No. 17. 



end of the external canal opens into the latter 

 at its middle back part. 



In the ampulla of the posterior semicir- 

 cular canal is a little circular group of minute 

 foramina, the inferior cribriform macula. 



The cochlea, named from its resemblance 

 to a snail-shell, is situated inwardly, and in 

 advance of the vestibule. It is a broad, low 

 cone, placed on edge, with its base applied 

 behind to the bottom of the internal auditor}^ 

 meatus, with its axis directed forward and a 

 little outward, and with its apex contiguous 

 to the eustachian tube and the bend of the 

 carotid canal. Externallj- it produces the 

 promontorj', and internally its wall is separated 

 from the exterior compact laj'er of the petrosa 

 bj- the spongj' substance occupying the inte- 

 rior of the apex of the latter. 



The cochlea consists of a cylindroid, slightly 

 tapering tube, the cochlear canal , which winds 

 spirally round a central column, named the 

 modiolus. The tube makes nearlj'' three turns, 

 gradually projecting in its course, and ending 

 in a rounded summit, the cupola. From the 

 round Tind oval windows, the cochlear canal 

 turns downward, inward, upward, and out- 

 ward, and continues in the same relative 

 course to the end. 



The modiolus, or central column of the 

 cochlea, is conical, with a broad base excavated 

 and impressed by the spiral tract at the bottom 

 of the internal auditory meatus, and with its 

 apex terminating immediatel}^ behind the end 

 of the cochlear canal. In the course of the 

 latter, the modiolus undergoes a rapid reduc- 

 tion, and, in the view of a longitudinal section 

 of the cochlea, appears as a short, wide, cj'lin- 

 drical column, with a second short and narrow 

 one projecting centrally from the former. 

 From the middle of the modiolus, in the course 

 of the cochlear canal, there projects a thin 

 shelf, named the spiral lamina. This reaches 

 about halfway across the canal, partiall}' divid- 

 ing it into two passages. lu the complete 

 condition of the labyrinth, a membranous tube, 

 the cochlear duct, extends along the cochlear 

 canal, between the spiral lamina and the op- 

 posite wall, and completely' separates the two 

 passages. Of these, one communicates with 

 the round window of the t3-mpanum, and is 

 hence called the tympanic passage,^ while the 

 other communicates with the vestibule, and 

 is named the vestibular passage.^ The two 

 passages communicate with each other at the 

 summit of the cochlea, within the cupola, by 

 a common orifice.^ 



The turns of the cochlear canal being; contig- 



Scala vestibuli. 



Hclicotrema. 



uous, in a longitudinal section of the cochlea, 

 thej' appear separated bj' a partition extending 

 from the modiolus to the pei'ipheiy of the coch- 

 lea and gradually thickening as it approaches 

 the latter. The partition is thickest at its com- 

 mencement, and graduallj' becomes thinner in 

 its course, until it abruptly terminates in a cres- 

 centic edge extended between the apex of the 

 modiolus and the cupola. The interior surface 

 of the cochlear canal, exclusive of the modiolus 

 and spiral lamina, is imperforate and smooth. 

 Opposite the spiral lamina it is commonly 

 marked bj- a faint line, indicating the attach- 

 ment of the spiral ligament. 



The surface of the modiolus curves continu- 

 ouslj' from this into the surfaces of the cochlear 

 cana],and spiral lamina. In transverse section 

 the canal appears more or less reniform. 



The spiral lamina is widest at its commence- 

 ment, opposite the round window, gradually 

 narrows in its course, and ends in a hook-like 

 process^ projecting from the apex of the modi- 

 olus. At its commencement a narrower por- 

 tion - is continued around to the opposite side 

 of the cochlear canal, where it arches over the 

 round window. The anterior surface of the 

 spiral lamina is directed into the vestibular 

 passage. A groove along its middle divides 

 it into two zones, of which that next the free 

 edge is the more compact and even. The pos- 

 terior surface looks into the tj'mpanic passage. 

 The free edge is rounded and miuutelj* serrulate. 



The surfaces of the modiolus and spiral lami- 

 na are minutely porous for the transmission of 

 vessels and nerves ; and this condition is more 

 marked within the tympanic passage. 



Commonlj' a row of larger elliptical foramina, 

 or pits, is situated within the latter passage, 

 along the root of the spiral lamina, extending 

 on the modiolus, giving this position a fluted 

 appearance. The arrangement is of variable 

 regularitj', sometimes interrupted, and at times 

 obscure. A narrower row of smaller and 

 more numerous foramina occupies the base of 

 the modiolus within the same passage. A row 

 of small foramina is also variably conspicuous 

 at the bottom of the modiolus, contiguous to, 

 the spiral lamina in the vestibular passage. 

 Within this, also, the modiolus is more or less 

 marked by miuute radiatiug grooves, which ad- 

 vance and branch on the anterior surface of the 

 passage. 



The modiolus is composed of fine spongy- 

 substance defined b^' a thin, more compact 

 layer. It is traversed b^' a central canal, 

 for the transmission of an arter3', commencing 

 at the central aperture of the spiral tract, and 



1 Hamulus. - Lamina spiralis secundaria. 



