OF HEARING. 



33 



gristly expansion, in the form of a horn or a funnel, the ob- 

 ject of which is to collect the waves of sound ; for this rea- 

 son, animals prick up their ears when they listen ; and for 

 the same reason, persons who are hard of hearing, em- 

 ploy an artificial trumpet, by which they may collect the 

 vibrations from a much more extended surface. The exter- 

 nal ear is peculiar to mammals ; and is wanting even in a 

 few aquatic species of these, such as the seals and the 

 Ornithorynchus. The ear of man is remarkable for being 

 nearly immovable. 



95. The middle ear has received the name 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 orbiculare (o) ; 

 which are articulated together, 

 as here represented, so as to form 

 a continuous chain. 



96. The internal ear, which is Fig. 20. 



also denominated the labyrinth, is an irregular cavity formed 

 in the most solid par^ of the temporal bone, beyond the 

 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 foramen rotundum, (Fig. 19, g), and the foramen 

 ovale (h). The first is closed by a membrane, similar to that 

 of the tympanum, while the latter is closed by the stapes, 

 one of the little bones in the chamber. 



