DEVELOPMENT OF THE EAR. 383 



This area of contact is the fenestra vestibuli (fenestra meaning window) . 

 When the chain of bones vibrates back and forth, the motion of the stapes 

 is transmitted through the fenestra vestibuli to the perilymph, and waves 

 may pass up the scala vestibuli and down the scala tympani, stimulating 

 the nerves of hearing in the cochlear duct. The blind termination of 

 the scala tympani rests against the lateral wall of the vestibule where also 

 the bone fails to develop; the round fenestra cochleae is thus produced. 

 Its fibrous membrane may yield somewhat to the perilymph waves, thus 

 relieving tension in the cochlea. 



\ 



s.tr. 



- / 



t.b. 



FIG. 433. HORIZONTAL SECTION THROUGH THE EAR OF A HUMAN EMBRYO OF ABOUT 5 CMS. 



a,u., Auricle; au.t., auditory tube; ch.t., chorda tympani; d.c., cochlear duct; d.s.l., and d.s.p., lateral 

 and posterior semicircular ducts ; e.a.m., external auditory meatus; fa., facial nerve; f c., fenestra 

 cochleae; p.s., perilymphatic space; st., stapes; s.tr., transverse sinus; t.b., temporal bone. 



In Fig. 433 the fragments of the chain of bones together with neigh- 

 boring nerves are imbedded in a mass of mesenchyma. In a later stage 

 the outer end of the auditory tube expands, filling all the space between 

 the vestibule and the bottom of the meatus. Thus it forms the tympanic 

 cavity. It encounters the chain of bones and the chorda tympani, and 

 wraps itself around them so that they lie in its folds or plicae. Thus all 

 structures which extend into the tympanic cavity, or appear to cross it, 

 are covered with a layer of entodermal epithelium derived from the audi- 

 tory tube. The original contact between the ectoderm and entoderm 

 of the spiracular cleft forms only an insignificant part of the tympanic 

 membrane. The latter becomes greatly enlarged, extending somewhat 

 along the upper surface of the ectodermal auditory meatus. The portion 



