Semicircular 

 canals 



Nerve to 

 brain 



Cochlea 



Eustachian 

 tube 



■^ 



Passage from 

 outer air 



THE HUMAN EAR 



Stirrup 



A sound vibration of the air strikes the tympanum, or drum, and is transmitted through 

 a chain of tiny bones to the liquid filling the "labyrinth". Disturbances of the liquid 

 stimulate delicate nerve endings in the cochlea, and the nervous impulses ore trans- 

 mitted to special regions of the brain 



We can see the relationship between these two chemical senses and to 

 an organism's adjustments in various ways. Thus both pleasant food odors 

 and food tastes arouse the salivary reflexes. A blindfolded person, holding 

 his nose to prevent currents of air from passing through it, cannot distin- 

 guish ground coffee, for example, from sawdust, or vanilla flavor from 

 raspberry. When we speak of the taste of good food, we usually mean the 

 odor. Feelings of nausea and the act of vomiting may be started by dis- 

 agreeable odors. 



Sensitiveness to Light We value seeing perhaps more than our other 

 senses because it puts us "in touch" with more of the world — with much 

 of the world that we are, in fact, unable to touch directly. We are able, 

 however, to understand that seeing depends upon chemical changes in the 

 sensitive structures — in the pigments that characterize all light-sensitive or- 

 gans. The source of the light, the objects that reflect the light by which we 

 see, may be very far away. The action on the nerve-endings, however, is 

 very close by, just as close as in the case of odor and taste or as in the case 

 of an actual push! 



289 



