PHYSIOLOGY 



187 



canal, leading into the middle ear within the skull. The pinna serves 

 to direct the sound waves to the meatus. The internal end of this 

 canal is covered by a thin, taut membrane which is called the tym- 

 panic memhrane or eardrum. This membrane is set into vibration by 

 the sound waves which reach it. The middle ear or tympanic cavity 

 is just internal to the eardrum and within temporal bone. It houses 

 three small bones or ossicles, the malleus (hammer), the incus (anvil), 

 and the stapes (stirrup). The malleus is attached to the eardrum and 

 the three bones are in contact with each other in such a way that they 

 conduct the sound vibrations through this cavity to a membrane at 

 the entrance to the inner ear, or fenestra ovalis, A tube, called the 

 Eustachian tube, leads from the cavity of the middle ear to the 



Fig-. 56. — Diagram of a section through the right human ear. B, semicircular 

 canal; G, external auditory meatus; o, oval window (fenestra ovale) ; P, tympanic 

 cavity containing the three auditory ossicles ; Pt., scala tympani ; r, round window 

 (fenestra rotunda) ; below r is seen the Eustachian tvibe ; S, cochlea; T, membrana 

 tympani ; Vt., scala vestibuli. (From Zoethout, TexthooTc of Physiology, The C. V. 

 Mosby Company, after Czermak.) 



pharynx, and it serves in the transmission of air between the cavity 

 and the exterior in order that the pressure may be kept at a balance 

 on the two sides of the tympanic membrane. This avoids interference 

 which might be caused by undue stretching of the membrane with 

 changes in barometric pressure. 



The internal ear contains the sensory apparatus, both for reception 

 of sound waves and for equilibrium. The organ of Corti in the 

 cochlea performs the first function and the semicircular canals, the 

 latter. The cochlea is a coiled tube filled with perilymph, a fluid, 



