NEURO-MUSCULAE MECHANISM 139 



simple organisms, devoid of any special organ of hearing, may 

 be affected by vibratory changes, and in fish it is difficult 

 to be certain how far such vibrations produce their effect 

 through the ear or through the body generally. But in higher 

 vertebrates it is chiefly through the ears that they act. In 

 these there is a special arrangement 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 (fig. 68). 



ZxM 



Fro. 68. Diagram of the Ear. Ex.M., external meatus ; Ty., tympanic mem- 

 brane ; m., malleus; i., incus; s., stapes; f.o., fenestra ovalis ; f.r. t 

 fenestra rotunda ; En.T., Enstacbian tube ; v., vestibule with the utricle 

 and saccule ; s.c., semicircular canal ; Coch., cochlea. 



The importance of such a mechanism in the anterior part 

 of the animal in warning it of danger or making it aware of the 

 presence of its prey is manifest. 



In mammals the organ of hearing consists of an external, 

 a middle, and an internal ear. The first is to conduct the 

 vibrations of the air to the second, in which these vibrations 

 produce to and fro movements of a bony lever, by which the 

 fluid in the third is alternately compressed and relaxed. 



2. External Ear 



The structure of this presents no point of special physio- 

 logical interest. In lower animals the pinna is under the 

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

 from which sound comes. 



