THE EAR 1 561 



■division of the auditor^" nerve lies in the internal auditory meatus after the 

 •comDletion of ossification. 



The membranous labyrinth, hitherto considered, is entirely epithelial 

 ■Certain of its epithelial cells undergo important speciahzations to fit them for 

 sensorial purposes. These cells form six groups in definite regions. These 

 groups are as follows: (i) The crista acustica ampullaris, of which there are 

 three, one in the' ampulla of each of the three epithelial semicircular canals; 

 (4) the macula acustica utriculi ; (5) the macula acustica sacculi ; and (6) the 

 organ of Corti. specialized from the epithelial ductus cochlearis. The groups 

 ■connected with the eimpullae of the semicircular canals, the utricle, and the 

 saccule, receive their nerv^e-fibres from the vestibular ganglion of the vestibular 

 division of the auditon,' nerve. The organ of Corti receives its ner\'e-fibres 

 from, the cochlear ganglion, or ganglion spirale, of the cochlear division of the 

 auditory ner\"e. 



Osseous Labyrinth. — The membranous lab\Tinth is surrounded by meso- 

 dermic tissue, which becomes disposed in four layers. These layers, from 

 -within outwards, may be-spoken of eis (i) the connective-tissue layer, (2) the 

 gelatinous layer, {3) the perichondrial layer, and (4) the cartilaginous layer. 

 The connective-tissue layer forms the connective tissue of the various parts 

 •of the epithelial lab\-rinth. The fibrous tissue of the gelatinous layer acquires 

 vacuoles, and gives place to the various perilymphatic spaces, which contain 

 a fluid called the perilymph. In the case of the cochlea the scala vestibuh 

 and scala tympani alone contain peril\Tnph, the scala nfiedia containing 

 •endolymph. The perichondrial layer forms the lining perichondrium of the 

 periotic cartilaginous capsule, and subsequently becomes the lining periosteum 

 -of the osseous labyrinth. The cartilaginous layer forms the periotic cartila- 

 ginous capsule, which undergoes ossification, and gives rise, amongst other 

 parts, to the osseous labyrinth, which is of larger size than the membranous 

 labyrinth. The osseous semicircular canals conform in shape to the mem- 

 branous semicircular canals, but are of larger size. The osseous vestibule 

 differs from the membranous vestibule in being a single osseous case, whereas 

 its contents are the utricle and the saccule. 



Whilst the perilymph hes external to the membranous labyrinth, the 

 interior of the semicircular canals, utricle, saccule, and scala media contain 

 endolymph, the latter being in communication with the saccule by means 

 of the canahs reuniens. 



Middle Ear and Eustachian Tube, — The middle ear, or tympanum, and the 

 Eustachian tube are developed from the entoderm of the upper end of the 

 first -visceral cleft, which is an evagination or diverticulum of the entoderm 

 of the phar>-ngeal portion of the alimentary tube. The entoderm of the 

 upper end of the cleft is elongated dorsalwards and out%vards. and its dorsal 

 •extremity, becoming expanded, forms the cavity of the middle ear, or tym- 

 panum, whilst the ventral portion of the entodermic diverticulum, remaining 

 tubular, gives rise to the Eustachian tube, which necessarily opens from the 

 pharynx. The entire entodermic diverticulum is invested by mesoderm ic 

 iissue. The tissue surrounding the enlargement which forms the tympanum, 

 as well as that surrounding the tympanic portion of the Eustachian tube, 

 forms part of the periotic cartilaginous capsule, and when ossified, constitutes 

 the osseous wall of the t)rmpanum, and the osseous upper part of the Eus- 

 tachian tube. The mesodermic tissue surrounding the lower part of the 

 Eustachian tube gives rise to the cartilage which characterizes that portion 

 of the tube. 



Inasmuch as the tympanum is developed from the upper end of the first 

 visceral cleft, it is intimately related to the upper ends of the first and second 

 visceral arches. From the upper end of the skeletal bar of cartilage (Meckel's 

 eartilage) of the first visceral, or mandibular, arch the malleus and the incus 

 are developed; and from the upper end of the skeletal bar of cartilage (hyoid 

 bar, or Reichert's cartilage) of the second visceral, or hyoid, arch the greater 

 part of the incus is developed. These ossicles — namely, the malleus, incus, 

 and stapes, constitute the tympanic ossicles of adult life. 



These three ossicles, together with the chorda tympani nerve, all lie external 



