47 HISTOLOGY 



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 plica. 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 

 of the malleus lying near it becomes imbedded in its mesenchymal layer, 



d.c.' 



FIG. 475. HORIZONTAL SECTION THROUGH THE EAR OF A HUMAN EMBRYO OF ABOUT 5 CMS. 

 au. f 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 acoustic meatus; fa., facial nerve; f.c., fenestra 

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



and its inner entodermal layer is made by the expansion of the tympanic 

 cavity. The jenlargement of the tympanic cavity continues after birth, 

 when it invades the spaces formed within the mastoid part of the temporal 

 bone. 



In spite of these modifications the course of the spiracular cleft is 

 retained in the adult. The ectodermal depression and its surrounding 

 elevations constitute the external ear; the pharyngeal outpocketing per- 

 sists as the auditory tube and the tympanic cavity of the middle ear. It 

 opens freely into the pharynx and contains air. 



SACCULUS, UTRICULUS, AND SEMICIRCULAR DUCTS. 



The walls of all these structures consist of three layers. On the out- 

 side there is connective tissue with many elastic fibers and occasional pig- 



