Chap. 17 



RESPONSIVENESS THE SENSE ORGANS 



319 



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Fig. 17.8. A flying bat makes an ultrasonic cry completely inaudible to human 

 ears. The curved lines represent the sound waves of a single pulse or vibration. Bats 

 emit as many as 50 of these sounds per second and locate obstacles to their flight 

 by hearing the echoes. The sound waves are here represented in true proportion to 

 the size of the bat. When a bat's ears are stopped it strikes whatever is in its path. 

 (Courtesy, Boring et al.: Foundations of Psychology. New York, John Wiley & 

 Sons, 1948.) 



auditory canal by a fleshy cover (tragus) that works like an eyelid. Man 

 and other primates have only hairs and wax to ward off insects, dust, and 

 water. The human ear has a cover at the entrance but has no means of pull- 

 ing it down. Neither human noses nor ears can close their doors. 



Each middle ear is an air-filled chamber opening into the pharynx by the 

 Eustachian tube. The middle ear contains a chain of three little bones: the 

 malleus or hammer at one end of the chain is attached to the eardrum by 

 ligaments; the incus or anvil is the middle link; and the stapes or stirrup at 

 the other end of the chain is attached to the membrane of the minute oval 

 window in the bony capsule containing the inner ear (Fig. 17.9). Sound 

 vibrations are transmitted from the eardrum over the bony bridge to the 

 internal ear. Under the impact of faint sounds the eardrum is tightened, and 

 under that of loud sounds it is loosened by involuntary muscles that attach 

 it to the bony bridge. Thus the bridge becomes a lever transmitting light or 

 heavy vibrations to the inner ear. Vibrations are also transmitted by the 

 surrounding bone. 



The inner ear contains the real mechanism of hearing, the organ of Corti, 

 triply protected by the membranous cochlea or cochlear duct, by surrounding 

 fluids, and by a casing of the hardest bone in the body (Fig. 17.9). The 

 structures of the cochlea are well known, but the details of the way in which 

 they work are still explained only theoretically. Only a bare outline of it can 

 be given here; books containing further details are given in the suggested 

 reading for this chapter. 



The cochlea is divided into three fluid-filled cavities, the cochlear duct, 



