THE EAR AND THE SENSE OF HEARING 131 



bases but leaning toward each other to form an arched 

 roof or tunnel. Toward the apex of the cochlea the rods 

 increase in length but are more widely separated at the 

 base, so that the tunnel becomes lower and wider. 

 Against the rods lean other cells called hair cells, which end 

 in many long, haiiiike processes. Between the hair cells 

 lie certain elongated supporting cells. The nerve fibers 

 end in fine branches between and around the hair cells. 



177. Path of an Auditory Impression. Sound waves pass 

 through the air and fall upon the membrane of the tym- 

 panum. In the middle ear they travel partly through 

 air and partly through solid bodies membranes and 

 bones, and in the inner ear through fluids and mem- 

 branes. Vibrations of the membrane of the tympanum 

 are u damped " by the ossicles of the middle ear, which 

 also receive and pass on the auditory tremors to the mem- 

 brane closing the oval window. These bones are so closely 

 bound together that they vibrate as if they were one, the 

 very slight amount of play at the articulations serving to 

 prevent jar and fracture. 



From the middle ear vibrations pass to the inner ear 

 through the attachment of the stapes to the membrane of 

 the oval window. Movement of that membrane sets up 

 motion in the fluid filling the cavity. That, however, 

 would not be possible (since the fluid is inelastic and 

 incompressible) Avere not a vent provided at the round 

 window. When the stapes pushes in the membrane of 

 the oval window, that of the round window bulges out- 

 ward, and the action agitates the whole body of the fluid 

 which fills the bony labyrinth. But the vibrations in the 

 fluid are also communicated to the walls of the membra- 

 nous labyrinth which it bathes, and the fluid which the 

 latter contains is thereby set in motion. 



