Chap. 17 RESPONSIVENESS THE SENSE ORGANS 317 



into the utricle there is a bulbous enlargement containing sensitive "hair 

 cells" like those in the utricle and saccule but without otoliths. These cells 

 are stimulated by movements of the fluid (endolymph) contained in the 

 canals and their tips are lurned in whatever direction it flows. Since the 

 canals lie in three diff"erent planes, a movement of the head in any direction 

 will afi'ect one or more of them. If the outer ear is irrigated with cold water, 

 convection currents are set up in the fluid of the canals and the person 

 will turn dizzy although his head and body may be kept upright and quiet. 

 The receptors in the canals may be strongly afi'ected by vertical movements 

 like that of an elevator, and by the rolling motions of ships and planes. 



Hearing — Phonoreceptors 



Ears and eyes work together so closely that we scarcely realize which one 

 brings the news. The eye sees in straight lines in light but the ears hear in 

 light and dark, and around the corner. The eye sees what something is; the 

 ear hears what it does. 



Probably relatively few invertebrates can hear. Insects that from very 

 ancient times have been land residents are the great exceptions; their per- 

 ception of vibrations of air is widespread. Fine hairs commonly borne on 

 the antennae of mosquitoes respond to vibrations. Crickets, cicadas, grass- 

 hoppers have tympanal organs or "eardrums" that vibrate in response to 

 the various clicks and squeaks which insects make with their legs and wings. 



In the vertebrates, the ears developed and took on their special function 

 as these animals gradually assumed their life on land. The new organ of 

 hearing developed from the saccule which retains its connection with the 

 semicircular canals of equilibrium, but the functions of the old and new 

 organs remained distinct. 



The Ear of Mammals — Man. The ears of mammals respond to vibrations 

 that are transmitted through air; this contains comparatively few particles so 

 that the vibrations or sound waves are relatively slow and widely diffused. 

 There is a great advantage in having the outer ear spread out to catch the 

 vibrations and a corridor to conduct them to the middle ear, from which they 

 are transmitted to the real sound receptors in the inner ear (Figs. 17.6, 17.7). 



The outer ear includes the outgrowth of flesh called the auricle and the 

 passageway to the eardrum, the auditory canal. Auricles are more or less 

 trowel-shaped and well supplied with cartilage and muscle. Sounds are lo- 

 cated by the intensity of the sound waves that stimulate the receptors of the 

 inner ear. We turn our heads and cup our ears to catch more sound waves, 

 as dogs turn their heads and lift both ears or one toward the sound. The 

 ear catches the sound wave; the brain decides where it comes from and 

 what it is. Human auricles are almost immovable; we cannot prick them 

 forward and backward with the attention that is so becoming to dogs and 



