72 SPECIAL SENSES. 



of hearing. The true seat of that sense is in the interior of 

 the head. It is usually a very complicated apparatus, espe- 

 cially in the superior animals. In mammals it is composed of 

 three parts ; the external ear, the middle ear, and the internal 

 ear, as shewn in fig. 45. 



148. The external ear consists of the conch (), and the 

 canal which leads from it, the external auditory passage (c, rf). 

 The first is a gristly expansion, in the form of a horn or a 

 funnel, the object of which is to collect the waves of sound ; 

 for this reason, animals prick up their ears when they listen. 

 The ear of man is remarkable for being nearly immoveable ; 

 therefore, persons whose hearing is deficient employ an arti- 

 ficial trumpet, by which they collect vibrations from a much 

 more extended surface. The external ear is peculiar to mam- 

 mals, and is wanting even in some aquatic species, such as 

 the seals and the ornithorynchus. 



149.. The middle ear has received the name of the tym- 

 panic cavity (h}. It is separated from the auditory passage 

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

 thoughit still communicates with the open air by means of anar- 

 row canal, called the Eustachian tube (k), 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 distin- 

 guished by the names of malleus (fig. 49, ), incus (6), stapes 



P. , c (d), and os orbiculare (c] ; which are articulated 

 rig. 4b. t v / ,. 



together, to form a continuous chain 



[The malleus, or hammer (fig. 46), has a rounded 

 head (1), a smooth articular surface connected by 

 a short neck (2) with the shaft of the bone, 

 which has a short process (3). The shaft or 

 handle (4) is lengthened and curved, and from 

 the front thereof proceeds a long delicate pro- 

 Fig. 47. cess (5). 



The incus, or anvil (fig. 4 7), resembles a bicuspid 

 tooth; its head (l)is hollowed out to receive the 

 head of the malleus ; the short process (2) serves 

 for the attachment of a ligament ; and the long 

 process (3) for its articulation with the orbicular 

 bone, which is early soldered to it. 



The stapes, or stirrup (fig. 48), is placed horizontally, with its 

 base resting upon the foramen ovale, and its head articulated 



