672 THE SENSE OF HEARING. 



image seen by the right eye, the other that seen by the 

 left (for example, the drawing of a cube A, B, fig. 188), be 

 presented to corresponding parts of the two retinae, as 

 may be readily done by means of the stereoscope, an instru- 

 ment invented by Professor Wheatstone for the purpose, 

 the mind will perceive not merely a single representation 

 of the object, but a body projecting in relief, the exact 

 counterpart of that from which the drawings were made. 



SENSE OF HEARING. 



Anatomy of the Organ of Hearing. 



For descriptive purposes, the ear, or organ of hearing, 

 is divided into three parts, the external, the middle, and the 

 internal ear. The two first are only accessory to the third 

 or internal ear, which contains the essential parts of an 

 Fig. 189.* organ of hearing. The accompany- 



ing figure shows very well the 

 relation of these divisions, one to 

 the other (fig. 189). 



The external ear consists of the 

 pinna or auricle, and the external 

 auditory canal or meatus. The prin- 

 cipal parts of the pinna are two 

 prominent rims enclosed one within 

 the other (helix and antihelix}, and 

 enclosing a central hollow named 

 the concha; in front of the concha, 

 a prominence directed backwards, 

 the tragus, and opposite to this, one 

 directed forwards, the antitragus. From the concha, the 



* Fig. 189. Outer surface of the pinna of the right auricle. . i, 

 helix; 2, fossa of the helix; 3, antihelix; 4, fossa of the antihelix ; 

 5, antitragus ; 6, tragus ; 7, concha ; 8, lobule. 



