198 



FOUNDATIONS OF BIOLOGY 



the jaws with the skull! Finally, the resonating (tympanic) 

 chamber recedes farther below the surface and becomes the 



Fia. 110. Front view of the human organ of hearing, right side, a, pinna of outer 

 ear; b, bone of skull; c, d, I, transmitting mechanism of three bones malleus, incus, 

 and stapes; e, one of the three semicircular canals; g, vestibule; h, auditory nerve; i, 

 cochlea; /, Eustachian tube leading to the throat; k, tympanic chamber or middle ear; 

 I, stapes; TO, tympanic membrane; n, external auditory passage, or outer ear; o, 

 cartilage. 



MIDDLE EAR to which sound waves are conducted through a 

 tubular passage, the OUTER EAR. In some forms, as in Man, 

 there is an external funnel-like collecting appendage, the 

 PINNA. (Fig. 110.) 



5. The Eye 



The organs of sight are the most complex sense organs of 

 animals and reach a very high degree of specialization even 

 in some of the Invertebrate forms. Among the latter the 

 essential sensory element (RETINA) of the eye usually arises 

 by the invagination of a limited area of ectoderm, the cells 

 of which become differentiated for receiving the photic 

 stimuli that produce impulses to be transmitted to the central 

 nervous system. Among Vertebrates the sensory cells are also 

 of ectodermic origin, but only secondarily so, since the OPTIC 



