404 STRUCTURE OF ORGAN OF HEARING. 



513. In animals which have the organ of hearing con- 

 structed upon the simple plan just described, the force of the 

 vibrations of the fluid contained in the cavity is increased by 

 several minute stony concretions suspended in it : these act 

 according to the second principle just stated ( 511). They 

 are termed otolithes, or ear-stones ; and some traces of them 

 may be found even in Man and the higher animals. 1 



514. We see, then, that a cavity excavated in the solid 

 walls of the head, covered-in externally by a membrane, 

 having the auditory nerve distributed upon its walls, and 

 filled with fluid, is the simplest form of the organ of hearing ; 

 and may be regarded as including all that is essential to the 

 exercise of this function. No more complicated apparatus is 

 to be found in any of the Invertebrata ; and even in the 

 lowest Fishes there is but little variation from this type. On 

 the other hand, in Man and the higher Vertebrata we find a 

 very complex structure, adapted to render the faculty much 

 more perfect ; enabling us to receive impressions which make 

 us. aware, not only of the presence of a sounding body, but of 

 its nature, its direction, the pitch and peculiar quality of the 

 sound ; and also, it is probable, taking cognizance of sounds 

 much fainter than those which would be perceptible to the 

 lower animals. Yet even in the most complicated forms of 

 the organ of hearing, we shall find that the essential part is 

 still the same as that which forms the whole organ in the 

 lower tribes ; and also that the faculty is retained, though in 

 an inferior degree, when by disease or injury the accessory 

 parts are prevented from acting. To the structure of the Ear 

 of Man we shall now proceed. 



515. The organ of hearing in Man may be divided into 

 three parts the external, the middle, and the internal ear. The 

 former is the fibro-cartilaginous appendage placed on the out- 

 side of the head, to receive and collect the sounds which are 

 to be transmitted to the interior ; the two latter divisions are 

 excavated in a bone of remarkable solidity, the petrous (stony) 

 portion of the temporal bone. The uses of the different 



1 Vesicles containing otoliths which are kept in rapid movement 

 within them by ciliary action, are found in immediate contiguity with 

 the cephalic ganglia of the lower Mollusks, or are even imbedded in 

 their substance; and these seem to constitute the most rudimentary 

 form of an organ of hearing. 



