OF HEARING 57 



gristly expansion, in the form of a norn or a funnel, the 

 object of which is to collect the waves of sound ; for this 

 reasor. , animals prick up their ears when they listen. The 

 ear of man is remarkable for being nearly immovable. 

 Therefore, persons, whose hearing is deficient, employ an 

 artificial trumpet, by which the vibrations from a much 

 more extended surface may be collected. The external 

 ear is peculiar to mammals, and is wanting even in some 

 aquatic species of these, such as the seals and the Orni- 

 thorhyncus. 



95. The middle ear has received the najme of the tym- 

 panic cavity, (k.) It is separated from the auditory passage 

 by a membranous partition, the tympanum or drum, (c ;) 

 though it still communicates with the open air by means 

 of a narrow canal, called the Eustachian tube, (i,) which 

 opens at the back part of the mouth. 



In the interior of the chamber are 

 four little bones, of singular forms, 

 which anatomists have distinguished 

 by the names of malleus, (Fig. 20, c,) 

 incus, (n,) stapes, (s,) and os orbicu- 

 lare, (o;) which are articulated to- 

 gether, so as to form a continuous 

 chain, as here represented, magnified. 



96. The internal ear, which is 



also denominated the labyrinth, is an irregular cavity formed 

 in the most solid part of the temporal bone, beyond trie 

 chamber of the middle ear, from which it is separated by a 

 bony partition, which is perforated by two small holes, called, 

 from their form, the round and the oval apertures, the fora- 

 men rotundum, (Fig. 19, g,) and the foramen ovale, (A.) The 

 first is closed by a membrane, similar to that of the tympa- 

 num, while the latter is closed by the stapes, one of the little 

 bones n the chamber. 



