:k;l' 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY 



B. The Ear and Hearing. 



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

 venient Iv 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 ('(implex form which receives the peripheral terminations of the 

 auditory nerve. This sac, therefore, is t<> 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. 179), is a peculiarly wrinkled sheet of tissue, consisting essen- 



Fig. 179.— Diagram of organ of hearing of lefl side (Quain, after Arnold): l. the pinna; 2. bottom of 

 concha ; 2-2', meatus externus; 3, tympanum ; above 8, the chain of ossicli - : •'•'. opening into the mastoid 

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



ced "ii the vestibule of the labyrinth above the fenestra ovalis; a, apex of the petrous I e; b, 



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

 mastoid process ; /, squamous pari of the bone. 



tiallv <it' yellow elastic cartilage covered with skin, and forming at the entrance 

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



The <-<ni<-ha y 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 ; lint in many of the lower animals the concha is relatively larger than 

 in man, and. their car- 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 



