166 THE THIRD DAY. [CHAP. 



obliterated at an early date, but from the inner part of 

 the cleft a diverticulum is given off towards the ex- 

 terior, which becomes the tympanic cavity. The inner 

 part of the cleft itself forms the Eustachian tube, while 

 its mouth forms the oral aperture of this tube. 



The meatus auditorius externus first appears as a 

 shallow depression at the region where the closure of 

 the first visceral cleft takes place. It is in part formed 

 by the tissue surrounding this depression growing up in 

 the form 'of a wall, but the blind end of the meatus also 

 becomes actually pushed in towards the tympanic 

 cavity. 



The tympanic membrane is derived from the tissue 

 which separates the meatus auditorius externus from 

 the tympanic cavity. This tissue is obviously consti- 

 tuted of an hypoblastic epithelium on its inner aspect, 

 an epiblastic epithelium on its outer aspect, and a layer 

 of mesoblast between them, and these three layers give 

 rise to the three layers of which this membrane is 

 formed in the adult. During the greater part of foetal 

 life it is relatively very thick, and presents a structure 

 bearing but little resemblance to that in the adult. 



The tympanic cavity is bounded on its inner aspect 

 by the osseous investment of the internal ear, but at 

 two points, known as the fenestra ovalis and fenestra 

 rotunda, the bone is deficient and its place is taken by 

 a membrane. 



These two fenestrse appear early, and are probably 

 formed by the nonchrondrification of a small area of 

 the embryonic cartilage. The upper of the two, or 

 fenestra ovalis, contains the base of a bone, known as 

 the columella. The main part of the columella is 



