THE SENSE OF HEARING. 807 



B. THE EAR AND HEARING. 



Anatomy and Histology of the Ear. The organ of hearing may con- 

 veniently be divided into three parts : (1) The external ear, including the 

 pinna or auricle and the external auditory meatus ; (2) the middle ear, called 

 the " tympanic cavity " or tympanum ; and (3) the internal ear, or labyrinth. 

 The labyrinth is situated in the dense petrous bone, and it contains a mem- 

 branous sac of complex form which receives the peripheral terminations of the 

 auditory nerve. This sac, therefore, is to the ear what the retina is to the eye ; 

 as the lens, cornea, etc. of the eye are simply physical media for the production 

 of sharp images on the retina, so all parts of the organ of hearing are devoted 

 solely to the accurate transmission of the energy of air-waves to the internal 

 ear. 



The External Ear. The pinna or auricle, commonly known simply as 

 the " ear" (Fig. 266), is a peculiarly wrinkled sheet of tissue, consisting essen- 



FIG. 266. Diagram of organ of hearing of left side (Quain, after Arnold) : 1, the pinna; 2, bottom of 

 concha ; 2-2', meatus externus ; 3, tympanum ; above 3, the chain of ossicles ; 3', opening into the mastoid 

 cells ; 4, Eustachian tube ; 5, meatus internus, containing the facial (uppermost) and auditory nerves ; 

 6, placed on the vestibule of the labyrinth above the fenestra ovalis ; a, apex of the petrous bone ; 6, 

 internal carotid artery ; c, styloid process ; d, facial nerve, issuing from the stylo-mastoid foramen ; e, 

 mastoid process ; /, squamous part of the bone. 



tially of yellow elastic cartilage covered with skin, and forming at the entrance 

 of the auditory meatus a cup-shaped depression called the " concha." 



The concha, and to some extent the whole auricle, serves a useful purpose 

 in collecting, like the mouth of a speaking-trumpet, the waves of sound falling 

 upon it; but in many of the lower animals the concha is relatively larger than 

 in man, and, their ears being freely movable, the auricle becomes of greater 

 physiological importance. 



External Auditory Meatus. In man the external auditory meatus or audi- 

 tory canal is about one and a quarter inches in length, and it extends from 



