DEVELOPMENT OF THE EAR 



1097 



Internal ear. — The process of development is as follows (fig. 843-845) : — 

 By invagination from the surface, an epithelial-lined vesicle, termed the primitive otocyst 

 or otic vesicle, is formed dorsal to the extremity of the second branchial cleft. It is at first 

 merely a pit on the surface, but eventually it loses its connection with the surface epithe- 

 lium and sinks into the interior. It then undergoes the alterations in shape and form shown 

 in the accompanying fig. 845. The vesicle is at first somewhat oval, and a small hollow 

 stalk arises from it, the recess of the labyrinth, which forms the ductus endolymphaticus in 

 the adult. The ventral and dorsal portions of the cyst become enlarged. From the former two 

 liollow plate-hke projections arise, one placed vertically, the other horizontally, and along the 

 free margins of these plates are formed the semicircular ducts, the superior and posterior from 

 the vertical, and the lateral duct from the horizontal one. The central part of each plate be- 

 comes peforated, and the periphery is thus altered to the characteristic loop form of the adult 

 semicircular ducts. The portion of the vesicle lying between the dorsal and ventral enlarge- 

 ments forms the primitive atrium. It becomes divided into two chambers, an upper dorsal 

 connected with the semicircular ducts, forming the utricle, and an inferior ventral, the saccule, 

 which is connected with that portion of the ventral expansion from which the cochlea is formed. 

 The recess of the labyrinth retains its connection with the cavity of the vesicle at the narrow 

 stalk connecting utricle and saccule, (fig. 845). The cochlea is formed by an outgrowth from 

 the saccule, at first straight, and later coiled in the fashion formed in the adult. 



Fig. 845.- 



-DiAGRAMS Illustrating Successive Stages in the Development of the 

 Membranous Ear. 



Semicircular 

 canal 



Sem icircul ar canals Ductus endolym- 

 phaticus 



Ductus utric- 

 lo saccularis 



Cochlear pouch 



Ductus reuniens 



Cochlea 



External and middle ear. — The external auditory meatus is formed from the dorsal part 

 of the first (external branchial) pouch, and the tympanic membrane from the membrane 

 which forms the floor of that pocket and separates it from the corresponding pharyngeal 

 (internal) pouch. Its outer surface is thus formed from ectoderm and the inner from endoderm. 

 The internal (pharyngeal) groove gives origin to the tympanic cavity and tuba auditiva, 

 the margins of the groove uniting. 



The auricle is formed from nodular thickening of the tissue bounding the outer end of the 

 first branchial cleft. Three nodules are formed on the first (mandibular) and three on the second 

 (hyoid) arch. Behind the latter, the free margin of the auricle is formed by a folding off of the 

 integument. Later an additional tubercle is formed dorsally between the two sets of nodules. 

 From the mandibular nodules are formed mainly the tragus and the crus of the helix — from 

 the hyoid tubercles the scaphoid fossa, antitragus and the crus of the anthelix. 



The auditory ossicles, and their muscles are formed from the neighbouring arches, the malleus 

 and incus, together with the tensor tympani, being derived from the first arch, while the stapes 

 and stapedius probably are derived from the second arch. 



The tympanic cavity is at first quite small, but later increases greatly, partly by the con- 

 densation of the loose areolar tissue which underlies its mucous membrane, the auditory ossicles 

 and their muscles being thus apparently brought within the cavity, and partly by the absorption 

 of the neighbouring bone. By this latter process the antrum and the tympanic and mastoid 

 cells are formed, all these depressions or cavities being hned by mucous membrane continuous 

 with that of the tympanic cavity. 



The Ear in the Child. — The ear in the newborn child shows several marked differences 

 from the adult ear. 



Among the principal differences are the following: — 



1. The external auditory meatus is very short, since the bony portion is undeveloped, 

 and is represented only by the tympanic ring. As a result of this, the tympanic membrane is 

 placed on a level with the surface of the head, and looks very much downward. 



2. The mastoid or tympanic antrum is relatively very large, and lies above and behind the 

 tympanum. Its lateral wall is only about 1 mm. in thickness. 



3. The mastoid process is not developed, and hence the stylomastoid foramen opens on 

 the surface behind the lower part of the tympanic ring. The exit of the facial nerve is therefore 

 much more upon the surface, and higher up than in the adult. 



4. The auditory (Eustachian) tube is nearly horizontal in direction. 



5. The ossicles are of nearly the same size as in the adult. 



