THE EAR 255 



the air are transmitted by the tympanum, the ear bones, 

 the perilymph, the labyrinth and the endolymph. The 

 basilar membrane is a long strip of thin membrane, 

 broader at one end than at the other, coiled in a spiral 

 form, and stretched between the bony walls of the laby- 

 rinth. Its tension and width are so varied that some 

 portion of its length between the broad end and the 

 narrow tip will vibrate sympathetically with all rates 

 not lower than twenty per second and not higher than 

 thirty thousand. 



Sensation of sound. Standing upon the" basilar 

 membrane are various complicated structures. Prom- 

 inent among these are certain slender highly sensitive 

 cells, the auditory cells, in which the fibres of the audi- 

 tory nerve start. Whenever any part of the basilar 

 membrane is set into vibration, the auditory cells of 

 that part are stimulated to activity. As a result, nerv- 

 ous impulses start inward along the nerve fibres of 

 the auditory nerve to the brain cells in the cortex, and 

 a sensation of sound results. If the vibrations of the air 

 are made up of a number of different rates of vibra- 

 tion, as is usually the case, not one but several regions 

 of the basilar membrane will be agitated, and a corre- 

 sponding number of nervous impulses and of blending 

 sensations will result. 



SENSE OF EQUILIBRIUM 



Semicircular canals. In close connection with the 

 internal ear are found the semicircular canals. In each 

 ear, there are three of these canals filled with fluid. 

 They are so arranged that any movement of the head 

 will cause movement of the fluid over the surface of the 

 -,wall containing it. This makes the fluid press upon 



