CHAPTER IV. 



DEVELOPMENT OF THE ORGAN OF HEAKING. 



The Structures which form the Organ of Hearing.— In 



figure 35 is shown diagrammatically the derivation of the five 

 elements which unite together to make up the organ of hearing. 

 The five elements are : 



(1) The otocyst — an area of epiblast (epithelial covering of 

 embryo) above the first branchial cleft which becomes invaginated 

 in a saccular form, and forms the epithelial lining of the mem- 

 branous labyrinth. Some of its lining cells become differentiated 

 into the auditory epithelium. 



(2) A ganglion derived from the " neural crest " of the hind 

 brain (Fig. 35). The nerve cells form the cochlear and vestibular 

 ganglia. Each cell sends out two processes, one to become 

 connected with the auditory epithelium of the otocyst, the other 

 to end in groups of nerve cells in the floor of the 4th ventricle, 

 their collective fibres forming the auditory nerve. The develop- 

 ment of the auditory nerve thus resembles that of the posterior 

 or sensory root of a spinal nerve. 



(3) The otocyst (membranous labyrinth) becomes surrounded 

 by a capsule of cartilage — the periotic capsule. This ossifies from 

 several centres, and forms the bony labyrinth and petro-mastoid. 



(4) The dorsal end of the first visceral cleft. — The inner 

 recess of the 1st cleft forms the Eustachian tube, the tympanum 

 and antrum of the mastoid ; the external cleft depression, the 

 external auditory meatus; while out of the cleft membrane is 

 formed the membrana tympani. 



(5) The malleus is derived from the upper end of Meckel's 

 cartilage, the incus from the posterior end of the palato-quadrate 



D 



