THE EAR 



91 



tion of the membranous labyrinth includes three semicircular canals, 

 respectively anterior, posterior, and lateral or horizontal, lying in 



planes perpendicular to each other, 

 so that movement in any direction 

 will cause a tendency to flow in 

 the contained fluid (the endo- 

 lymph) of one or more canals. 

 Each canal has at one point an 

 expansion, the ampulla, and all 

 connect with a larger sac, the 

 utriculus, the endings of the ves- 

 tibular nerve fibres being in the 

 walls of these. The utriculus con- 

 nects with the endolymphatic duct, 

 which runs to the cranial cavity 

 and ends in a blind sac within the 

 thickness of the dura mater. A 

 narrow passage connects the utricu- 

 lus, further, with another relatively 

 large space, the sacculus, which also 

 has vestibular nerve endings, and 

 this in turn is connected by a fine 

 tube with the cochlear duct, the 



external acoustic meatus, terminating at ,. • r i i 



the tympanic membrane; m.a.i., internal aUQltOry pOrtlOn 01 the mem bran - 



acoustic meatus; s., sacculus; s.e., endo- i i • ^i t^ ^i ^ j_i 



lymphatic sac; St., stapes; t.a., auditory ous labyrmth. It appears that the 



tube; u., utriculus; v, vestibulum; VIII, , • j_i • 11 .• 



acoustic nerve. rcccptors m the ampullae are Stimu- 



lated essentially by movement, 

 those in the utriculus and in the sacculus responding rather to 

 position (gravitational stimuli). 



While the vestibular parts of the labyrinth are completely 

 surrounded by fluid, the perilymph, in the bony canals, the cochlear 

 duct is attached to its bony enclosure along one side. It is also 

 connected with the opposite w^all by a membrane so that the cavity 

 within the bony cochlear canal is divided into three parallel tubes, 

 which are coiled in a close spiral and taper gradually towards the 

 apex. Within the cochlear duct (which, like other parts of the 

 membranous labyrinth, is filled with endolymph), is the complex 

 structure containing the actual auditory sensory nerve endings, 

 the organ of Corti. 



Fig. 47. Diagram of the parts of the 

 ear in vertical projection. To show the 

 general relations of the structures 

 covered by the dissection. 



p, petrous portion of the petrotym- 

 panic bone; t., tympanic portion (bulla 

 tympani). 



c, cochlea; c.s., bony semicircular 

 canals; c.t., tympanic cavity; d.c, coch- 

 lear duct; d.e., endolymphatic duct; d.m., 

 dura mater; d.s., semicircular ducts; f.c, 

 cochlear fenestra; f.v., vestibular fen- 

 estra; i., incus; m, malleus; m.a.e., 



