A STUDY OF THE SENSES 313 



thin partitions into three compartments ; the upper and lower 

 are filled with perilymph, the smaller middle one contains 

 endolymph. The latter portion of the cochlea is directly 

 connected with the cavity of the sacculus, and hence is in 

 communication with the utriculus and semicircular canals. 

 Sensory cells with long projecting hairs are found on the 

 floor of the central cavity, and to these spirally arranged 

 cells run fibers of the -auditory nerve. 



Sensations of Sound. When a stone is dropped into a 

 pond, ripples move outward in circular waves over the sur- 

 face, and finally disappear. In a similar manner sound 

 waves are transmitted in all directions from a given body, 

 say a bell that is struck and caused to vibrate. Some of 

 these waves of air enter the tube of the external ear, cause 

 the tympanic membrane to vibrate, and this in turn sets in 

 motion the chain of small bones that reach across the cavity 

 of the middle ear. The movements of the stirrup bone set 

 in vibration the thin membrane to which it is attached, and 

 so the perilymph which lies in the inner ear becomes dis- 

 turbed. Since the perilymph is continuous throughout the 

 cavities of the inner ear, the vibrations which have been set 

 up in the way we have described may be transmitted in all 

 directions throughout the bony labyrinth. 



It is probable, however, that these waves become most 

 effective as they move up through the coils of the cochlea, 

 and set in motion the thin partitions that inclose the middle 

 cavity. When the endolymph becomes disturbed, it moves 

 the hairs of the sensory cells, and thus an impulse is finally 

 started along the fibers of the auditory nerve. We get sen- 

 sations of sound when the brain cells receive and interpret 

 these impulses. 



Loudness, Pitch, and Quality. The various sounds of 

 which we are conscious differ in loudness, in pitch) and in 

 quality. If I tap a bell lightly, I cause its metal to vibrate 

 only a little; the air waves that influence my middle and 

 inner ear are feeble, and I can scarcely hear the sound. 



