COORDINATION 



213 



conducting and also for collecting and augmenting the sound waves. 

 The result is that, in ascending the Vertebrate series, we find the 

 ear proper receding farther and farther below the surface. 



Soon, between the labyrinth, or inner ear, and the surface of the 

 head, a simple resonating chamber is added which is provided with 

 a vibrating tympanic membrane, or ear drum, situated just under 

 the skin. Then this is improved by the development of a bony 

 transmitting mechanism between the tympanic membrane and the 

 inner ear. This consists of a single bone until we reach the Mam- 

 mals, when two more bones are added by being diverted from their 



Pinna 



Incus 



Malleus 



Semicircular 



canal •Vestibule 



Auditory nerve 



stachian 

 tube 



Auditory passage Stapes 

 or outer ear Tympanum 



Fig. 148. — Front view of a vertical section of the human ear, right side. 

 Note the transmitting mechanism of three bones: malleus, incus, and slapes. 



earlier function of articulating the jaws with the skull. Finally, 

 the resonating (tympanic) chamber recedes farther below the sur- 

 face and becomes the middle ear to which sound waves are con- 

 ducted through a tubular passage, the outer ear. In some forms 

 the latter is provided with an external funnel-like expansion, the 

 pinna. Apparently much is accomplished by accumulating minute 

 changes during the ages. (Fig. 148.) 



5. The Eye 



The organs of sight are the most complex sense organs of ani- 

 mals 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 



