SENSE ORGANS. 



175 



ter in direction at right angles to the drumhead, and 

 three quarters of an inch in a direction up and down. 

 It is connected with the throat by a slender tube — the 

 Eustachian tube — and is therefore air-filled. The closure 

 of this tube by inflammation of the throat is a frequent 

 cause of partial deafness. By holding the nose and 

 blowing hard, air may be forced through the Eustachian 



Fig. 109. — Simplified diagram representing a section through the ear ; the 

 cochlea is supposed to be unrolled : ma, meatus auditorius ; t, tympanic 

 membrane ; m, in, st, the ossicles ; sc, semicircular canals ; c, cochlea ; 

 scv, set, scala vestibuli and scala tympani ; eu, eustachian tube. The 

 shaded part represents bone. (From Huxley.) 



tube into the drum, and cause sensible pressure on the 

 membrane of the drum. 



This cavity is separated from the outer ear by the 

 membrana tympani, and from the inner ear by a bony 

 wall, in which are two openings closed with membrane, 

 viz., the foramen rotundum and foramen ovale. These 

 membranes act as counter-drumheads to the membrane 



of the drum. 

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