THE ORGAN OF HEARING. 



4OI 



phosed outer end of the first pharyngeal pouch, or inner visceral 

 furrow, the Eustachian tube representing the inner segment. 

 The tympanic membrane results from the changes affecting the 

 septum between the outer and inner first visceral furrows ; this par- 

 tition, originally thick, consists of a mesodermic middle stratum, 

 covered on its outer and inner surfaces respectively by the ecto- 

 derm and the entoderm. The external and the middle ear at no 

 time communicate, but are normally separated by the septum in 

 question. 



The ear-ossicles are developed in connection with the primitive 

 skeleton of the visceral arches ; the malleus and the incus rep- 

 resent specialized parts of the cartilaginous rod of the first arch. 

 The development of the stapes, on the contrary, is probably not 

 connected with the visceral skeleton, but owes its formation to the 

 ossification of the tissues at the fenestra ovalis. 



The development of the external ear results from the changes 

 taking place within the first outer visceral furrow, or gill-cleft, 

 and the tissue immediately around its external orifice. From 

 the gill-cleft originates the external auditory canal, and from the 

 margins of its orifice the pinna is formed. 



26 



