DEVELOPMENT OF THE ORGAN OF HEARING. 



51 



the meatal roof (Figs. 36 A and B). Over the roof lies the third 

 temporal convolution. 



/roof of meatus 



tegmen tympani 

 attic of tympanum 



f roof of meatus 

 petro-squamosal suture 

 tegmen tympani 



int. aud. meat. 



tympanic ring 

 fibrous plate 



meatus 



membrana tympani 

 iymoanie plate 



A B 



Fig. 36 A.— A Section of the External Auditory Meatus of the Adult. 

 Fie. 36 B. — A Section of the External Auditory Meatus at Birth. (After Symington.) 



The meatus is supplied in front by the nerve of the mandibular 

 arch (auriculo-temporal branch). Why the vagus should supply 

 it with a branch (Arnold's nerve) is obscure. The vagus is a 

 visceral nerve and supplies the 3rd and 4th clefts by its 

 superior and inferior laryngeal branches. In fishes a branch 

 of the vagus passes backwards beneath the skin on each side 

 and supplies the sense organs of the lateral line. Many regard 

 the auricular branch of the vagus as a vestige of such a branch. 



In the newly-born child the membrana tympani is so obliquely 

 set that its outer surface is almost in contact with the meatal 

 floor. With the development in length of the meatus, it becomes 

 more vertical in position. The meatus may be only partly 

 developed or even absent, the upper part of the 1st cleft 

 becoming completely closed like the lower part. In such a 

 case there is commonly a corresponding absence of development 

 of the middle and internal ear. 



The External Ear. — Six tubercles appear on the mandibular 

 and hyoid arches round the 1st cleft depression and form the 

 external ear (see Figs. 37 and 38). Two of these tubercles 

 grow from the mandibular arch and form the tragus and crus 

 of the helix ; three from the hyoid to form the lobule, antitragus 



