368 STRUCTURE OF THE COCHLEA. 



bright light falling upon the eye brings on a contraction of the pupil. 

 And so with the ear. A partial deafening must be established to adjust 

 the intensity of sound, that the auditoly nerve may act under standard 

 circumstances. The primary impression therefore made upon that nerve 

 by the loudness of sounds is, so to speak, consumed by being converted 

 as a reflex act into motion, because there is a necessity that the tensor 

 and stapedius should move, and reflex acts do not affect the mind, but 

 it instantly perceives the condition of contraction of those muscles, and 

 so estimates the intensity of the sound. 



2d. On the measurement of wave length, or time of vibration of sounds. 

 Structure of the cochlea and its functions. 



The structure of the cochlea is so significant that its true function 

 Structure of has been long ago more or less distinctly recognized. Thus 

 the cochlea, p^ Young speaks of it as a micrometer of sound. Many 

 physiologists regard it as determining the flote or pitch. Any one who 

 remarks the gradually decreasing width of its spiral lamina, and the 

 manner in which the ultimate filaments of the auditory nerve are spread 

 thereon, becoming shorter and shorter as they ascend the scale who re- 

 calls the structure of the harp, or gradually shorter strings of the piano- 

 forte, could scarcely fail of being impressed with the truth of this conclu- 

 sion. The function of the cochlea is the determining of wave length, 

 that is, the time of vibration or note of sounds. 



The cochlea has been described as resembling a snail's shell in appear- 

 ance. It is a conical tube, wound spirally, and making two and a half 

 turns. The interior of this conical and spirally- winding tube is divided 

 throughout its length into two portions by means of a transverse parti- 

 tion, which, following the spiral winding of the tube, has had the name 

 of lamina spiralis bestowed on it. The two partitions produced by the 

 intervention of this lamina are called scala vestibuli and 



The two scalce. . 



scala tympani. At the top or point of the helix the two 

 scalas communicate through a little hole, from the cessation of the lamina 

 spiralis. To this opening or deficiency the name of helicotrema is given. 

 Considering the two scalse as separate tubes, their mouths open differ- 

 ently ; the scala vestibuli opens into the vestibule of the labyrinth, and 

 we may therefore regard the membrane of the fenestra ovalis as being 

 virtually its boundary or closure, but the mouth of the scala tympani is 

 against the fenestra rotunda, and is closed by the membrane of that aper- 

 ture. As their names therefore indicate, the scala vestibuli opens into 

 the vestibule, and the scala tympani into the tympanum. 



Passing directly through the body of the cochlea, and being, as it 

 introduction were tne core upon which that structure is built, is a bony 

 of the auditory cone, called the modiolus. Indeed, the bony part of the 



transverse plate which separates the tube of the cochlea into 



