THE SPECIAL SENSES. 21? 



ear. The extreme limit of the capacity of the human ear 

 maybe considered as included between these points; but 

 the sounds which we ordinarily hear are embraced be- 

 tween 100 and 3,000 vibrations per second. 



89. The ear, which is the proper organ of hearing, is the 

 most complicated of all the structures that are employed 

 in the reception of external impressions. The parts of 

 which it is composed are numerous, and some of them are 

 extremely small and delicate. Nearly all these parts are 

 located in an irregularly shaped cavity hollowed out in the 

 temporal, or " temple," bone of each side of the head. 

 That part of the bone in which the auditory cavity is 

 placed has the densest structure of all bones of the body, 

 and has therefore been called the "petrous," or rocky part 

 of the. temporal bone. In studying the ear, it is necessary 

 to consider it as divided into three portions, which are 

 called, from their relative positions, the external ear, the 

 middle ear, and the internal ear. (In the diagram, Fig. 53, 

 A, the first is not shaded, the second is lightly shaded, and 

 the last has a dark background.) 



90. The External Ear. The external portion of the 

 organ of hearing, designated in Fig. 53, A, includes, first, 

 that outer part (a), which is commonly spoken of as "the 

 ear," but which in fact is only the portal of that organ ; 

 and, secondly, the auditory canal (b). The former consists 

 of a fiat flexible piece of cartilage, projecting slightly from 

 the side of the head, attached to it by ligaments, and sup- 

 plied with a few weak muscles. Its surface is uneven, and 

 curiously curved, and from its resemblance to a shell, it 

 has been called the concha. It probably serves to collect 

 sounds, and to give them an inward direction ; although 

 its removal is said not to impair the acuteness of hearing 

 more than a few days. 



9O. Of what docs the external portion of the organ of hearing consist? De- 

 scribe the portal of that organ known as the ear. Its use ? 



