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 Yertebrata 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. 
