COORDINATION 



405 



auditory nerve 



NNER EAR 



eustachian tube 



Fig. 16-9. The human ear dissected in order to show the various parts that have to do v/lth receiving sound and 



the part that is concerned with balance. 



termine the source of the sound. In such 

 forms where the appendages are especially 

 enlarged, they also function as temperature 

 regulating mechanisms, for their great size 

 and profuse vascularization provides an 

 ideal apparatus for cooling the blood. In 

 manj the function of the pinnae is dubious, 

 since auditory acuity is no greater in those 

 with generous pinnae than in those who 

 have small external ears. Furthermore, 

 movement of the ears is limited to a privi- 

 leged few, even though everyone possesses 

 a full set of muscles to accomplish the feat 

 (Fig. 25-14). The external ear surrounds 

 the opening which leads through the audi- 

 tory canal and terminates at the tympanum, 

 or eardrum. The walls of the canal are sup- 

 plied with glands that produce wax which 

 discourages small creatures such as insects 

 from entering. 



The middle ear consists of a chamber 

 connected to the pharynx by the eustachian 

 tube, an old gill pouch remnant. Bridging 

 across this air chamber is a series of three 

 bones (ear ossicles) which conduct vibra- 

 tions of the tympanum to the cochlea. Al- 



though the eustachian tube is advantageous 

 in equalizing the pressure on both sides of 

 the eardrum, it does have a disadvantage 

 in that microorganisms in the mouth can 

 make their way through this tube and infect 

 the middle ear region. Such infections can 

 be dangerous, sometimes leading to deaf- 

 ness. The tiny bones are named the ham- 

 mer, anvil, and stirrup ( malleus, incus, and 

 stapes, respectively) because of their 

 shapes (Fig. 16-9). Together they produce 

 a lever arm which diminishes the amplitude 

 of the tympanic vibrations but at the same 

 time intensifies them. The hammer is at- 

 tached to the eardrum and the anvil, while 

 the stirrup, attached to the anvil at its op- 

 posite end, fits into the oval window of the 

 cochlea. Vibrations conducted through the 

 chain of bones are conveyed to the liquid- 

 filled cochlea. Another membrane-covered 

 opening, the round window, allows the fluid 

 to vibrate freely, without being lost from 

 the closed chambers. Since liquids do not 

 compress, the vibrations retain all of their 

 vigor until they are delivered to the sensory 

 cells which generate impulses that are 



