456 Dynamic Theory. 



In the first place the external ear, or ear flap, is in man not essential, 

 or even helpful, to the sense of hearing, but is a mere rudiment. Nor 

 is the ear flap possessed by birds. The ear flaps with the tube called 

 the auditory canal, or external meatus, leading in from it, constitute 

 the external ear. At the inner end of the auditory canal is the tym- 

 panic membrane which is stretched across the opening, and forms its in- 

 ner terminus. Inside of the tympanic membrane is the chamber, which 

 with its contents constitutes the middle ear the tympanum, or drum. 

 The tympanic membrane is one head of this drum, the one that receives 

 the original vibrations. On the opposite side of the drum from this 

 head, are two small membranes, which separate the drum from the in- 

 ternal ear or third portion. This internal ear is the essential part, and 



. Fenestra Ovalis oval window. 

 . Fenestra Rotunda round window (in profile ). 

 . Three Semicircular Canals ; e. Ampullae. 



FIG. 192. Partly Diagrammatic Scheme of the Ear. 

 M. Meatus or external opening ; arrow pointing in. 

 Tm. Drum-head or tympanic membrane. 

 T. Drum or tympanic cavity or middle ear. 

 Et. Eustachian tube ; arrow pointing to the pharynx. 

 h. Hammer; a. Anvil; z. Orbicular bone. 

 s. Stirrup resting upon o. 

 o. Fenestra Ovalis oval window. 

 R. 

 c. 



Ut.The Utricle. 

 ,<?&. The Sacculus.' 

 V Vestibule. 

 vt. Scala Vestibuli, ) 

 pt. ' Tympani, > Cochlea. 



6.' Bone. (After Landois and Stirling.) 



like that of the simplest animals, consists primarily and essentially of a 

 water} T fluid suspended in and around membranes, upon which are ter- 

 minations of the auditory nerve. The watery fluid, or lymph, as it is 

 called, is contained in three subdivisions of the internal ear. The sim- 

 ple sac of the invertebrate ear is in the human ear, represented by 

 two sacs of very peculiar shape. One of these is called the seal a me- 

 dia, the other the membraneous labyrinth. The two are surrounded by, 



