240 



ELEMENTARY BIOLOGY 



284. Sound-perceiving organs. Among the insects there are 

 many special sound-producing organs, as well as many sound- 

 perceiving organs. It is probable also that many insects are 

 sensitive to rates of vibration to which our own nerve endings 

 are indifferent (see Fig. lOO). In some insects and spiders 



the sound waves are re- 

 ceived on fine stretched 

 hairs connected with 

 nerve fibers. In the 

 male mosquito and in 

 other insects sound 

 waves are received by 

 fine hairs on the an- 

 tennae, or feelers. 



285. The human ear. 

 In the air-breathing ver- 

 tebrates the hearing or- 

 gan is very much like 

 our own, although it is 

 possible to arrange a 

 series, extending from 

 the amphibians to the 

 mammals, in which in- 

 creasing complications 

 and refinements may be 

 observed. Our own hear- 

 ing organ is pictured in 

 Fig. lOi. 

 286. How the ear works. A vibration striking the ear drum 

 is transmitted through the chain of bones of the middle ear to 

 the liquid filling the labyrinth. From this liquid it is trans- 

 mitted to the delicate lining of the cochlea, where nerve-end- 

 ings are located. Here some cells are disturbed by vibrations 

 of one pitch, others by those of a different pitch. 1 he nerve 

 fibers are connected with special cells in the brain. 



Fig. 101. The human ear 



A, the outer ear, consisting of the cartilaginous pro- 

 jection from the side of the head and an air passage, 

 or vestibule 7' ; B, the middle ear, lying between 

 the ear drum, or tympanum (/), and the inner ear, C. 

 The inner ear is connected with the pharynx by the 

 Eustachian tube e (see Fig. 28). Extending from 

 the drum to the inner ear is a series of three tiny 

 bones: /;, the hammer-, «, the anvil \ and j, the 

 stirrup. The main parts of the inner ear constitute 

 the labyrinth : c, the semicircular canals, consisting 

 of three tubes placed almost exactly at right angles 

 to one another ; k, the cochlea, or snail shell. The 

 labyrinth is filled with a fluid and lined by a delicate 

 membrane containing nerve endings 



