250 OUTPOSTS OF THE INTELLIGENCE SERVICE 



2. Middle ear. The mechanism found m the middle ear converts 

 vibrations in air into vibrations in fluid by means of membranes 

 and a series of levers. It consists of an air-filled cavity hollowed 

 out of the petrous part of the temporal bone. It is separated from 

 the external car by the tympanic membrane, and from the internal 

 ear by the membrane closing the round window and by a disc of 

 bone — -the foot of the stapes, which along with the membranous 

 collar surrounding this bone makes a fluid-tight packing or gland 



Fig. fi3. — Diagrammatic view of auditory organ. (After Scliafer.) 

 1, Auditory nerve; 2, internal acoustic meatus; ;?, utricle: 4, semicircular duct; 

 .5, saccule; 6, .scala media of cochlea; 9, vestibule containing lymph; 10, semicircular 

 canal ; 12, stapes ; 13, fenestra rotunda ; 19, incus ; 18, malleus ; 17, niembrana tympani ; 

 16, external acoustic meatus ; 14, auricle or pinna ; 23, Eustachian tube. 



filling the foramen ovalis, the oval opening into the internal ear. 

 Between the drum and the stapes lie two bony levers — the malleus 

 and the incus. 



(a) Membrana tympani. This structure is fixed in a frame of 

 bone which is almost circular (vertical diameter 10 mm. ; hori- 

 zontal diameter 8-5 mm.). Although it is not more than 0-1 mm. 

 thick, it is constructed of three layers. On the outer surface 

 there is a layer of epithelium protecting the membrane proper, 

 which is of fibrous tissue and is covered on the inner side by a 

 layer of mucous membrane. The fibres of the fibrous layer are 

 arranged partly circularly and partly radially — the circular fibres 



