DEVELOPMENT OF THE EAR. 



1525 



Wall of 

 brain-vesicle 



Endolymphatic 

 recess 



Vestibular 

 pouch 



the epithelium within certain areas associated with the distribution of the auditory nerves. 

 The nerve-fibres form their ultimate relations with the sensory areas by secondary growth into 

 the epithelial structures. 



Development of the Auditory Nerves. — The vestibular and cochlear nerves, according to 

 Streeter\ develop from a ganglion-mass first seen at the anterior edge of the otic vesicle. This 

 consists of an upper and lower part 

 from the dorsal and ventral portion 

 of which peripheral nerve branches 

 are developed, whilst a single stem 

 connects it with the brain. 



The nerves destined for the 

 utricle and the superior and external 

 ampullae develop from the upper 

 part of the ganglionic mass, while 

 the nerves which supply the saccule 

 and posterior ampulla develop from 

 the lower part of this mass. The 

 stem extending centrally from the 

 ganglion toward the brain becomes 

 the vestibular nerve. 



The spiral ganglion begins its 

 development at the ventral border 

 of the lower part of this mass, the 

 cochlear nerve growing toward the 

 brain while the peripheral division 

 containing the ganglion extends into 

 the membranous cochlea. From 

 the foregoing sketch, it is evident 

 that the membranous labyrinth is 



genetically the oldest part of the internal ear, and that it is, in fact, only the greatly modified 

 and specialized closed otic vesicle surrounded by secondary mesoblastic tissues and spaces. 



Development of the 



Cochleai- 

 pouch 





r 



Otic vesicle shows differentiation into three subdivisions, endo- 

 lymphatic, vestibular and cochlear. X 40. 



Fig. 1279. 



,v.-::f-:^^^' 



Wall of brain- 

 vesicle, 



Endolymphatic duct 



Canalicular 



recess 



Utriculo- 

 saccular pouch 





Surface 







Cochlear duct 



^i^i^- 



Further difEerentiation of otic vesicle into endolymphatic duct, utriculo- 

 saccular pouch and cochlear duct. 



Middle Ear. — The tympanic 

 cavity and the Eustachian 

 tube are formed essentially 

 by the backward prolonga- 

 tion and secondary expansion 

 of the inner entoblastic por- 

 tion of the first branchial fur- 

 row, the pharyngeal pouch. 

 The dorsal part of the latter, 

 in conjunction with the adja- 

 cent part of the primitive 

 pharynx, gives rise to the sec- 

 ondary tiibo-tympanic space 

 (Fuchs); the posterior end 

 of this becomes dilated to 

 form the tympanic cavity, 

 while the segment interven- 

 ing between the tympanic 

 diverticulum and the pharynx 

 is converted into the Eusta- 

 chian tube. The first and 

 second branchial arches con- 

 tribute the roof of the tym- 

 panic cavity. 



The ear ossicles are de- 

 veloped in connection with 

 the primitive skeleton of the 

 visceral arches. The malleus 



and incus represent specialized parts of the cartilaginous rod of the first arch, the tensor tym- 

 pani being developed from the muscular tissue of the same arch. The stapes is developed from 

 the second arch. The mesoblast which surrounds the structures of the tympanic cavity during 

 their development becomes spongy and finally degenerates toward the end of foetal life. 



^Amer. Jour, of Anatomy, Vol. VI., 1907. 



