130 HUMAN PHYSIOLOGY 



the ears that they act. In these there is a special arrange- 

 ment by which the vibrations of the air are converted into 

 vibrations of a fluid in a sac situated in the side of the 

 head into which the free ends of neurons project. 



In mammals the organ of hearing consists of an external, 

 a middle, and an internal ear. 



A. External Ear. The structure of this presents no point 

 of special physiological interest. In lower animals the pinna 



LxM 



Fio. 68. Diagram of the Ear. ExM., external meatus ; Ty. , tympanic mem- 

 brane; m. , malleus; t., incus; ., stapes; /.o., fenestra ovalis; f.r., fenestni 

 rotunda; EnT., Eustachian tube; t>., vestibule; .c., semicircular canal; 

 Coch., cochlea. 



is under the control of muscles, and is of use in determining 

 the direction from which sound comes. 



B. Middle Ear. The object of the middle ear is to over- 

 come the mechanical difficulty of changing vibrations of air 

 into vibrations of a fluid. It consists of a chamber, the 

 tympanic cavity, placed outside of the petrous part of the 

 temporal bone (Fig. 68). Its outer wall is formed by a 

 membrane, the membrana tympani (Ty.), which is attached 

 to a ring of bone. Its inner wall presents two openings into 

 the internal ear the fenestra ovalis (/.o.), an oval opening, 

 situated anteriorly and above, and the fenestra rotunda (/.?*.), 

 a round opening placed below and behind. Throughout life 

 these are closed, the former by the foot of the stapes, which 



