DEVELOPMENT OF THE EAR. 



1525 



Endolymphatic 

 recess 



Otic vesicle shows differentiation into three subdivisions, endo- 

 lymphatic, vestibular and cochlear. X 40. 



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 1 , 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 p IG I2 y8. 



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 W al1 f 



,; j , r brain-vesicle 



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 



FIG. 1279. 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 tubo-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 



Wall of brain- 

 vesicle 



c duct 



alicular 



recess 



ar pouch 



face 



Cochlear duct 



Further differentiation of otic vesicle into endolymphatic duct, utriculo- 

 saccular pouch and cochlear duct. 



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 . 

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



