EAR 469 



ducts and their perilymph spaces, so that when the membranous labyrinth 

 which they form, is removed by maceration, the bone still contains a 

 corresponding arrangement of cavities and canals. These constitute the 

 bony labyrinth. Casts of it, made in soft metal, may be seen in all anatom- 

 ical museums. Instead of subdivisions to correspond with the utriculus, 

 sacculus, and utriculo-saccular duct, the bony labyrinth has a single space, 

 already referred to as the vestibule. Into it the semicircular and cochlear 

 canals open, together with the aquczductus vestibuli which contains the 

 endolymphatic duct. 



The middle ear and external ear arise in connection with the first or 

 spiracular gill cleft. In common with the other clefts, this includes an 

 entodermal pharyngeal outpocketing (Fig. 206, p. 217) and anectodermal 

 depression (Fig. 205, sp.}. At an early stage these meet one another and 

 fuse, but later, the primary epithelial connection breaks down, and 

 mesenchyma intervenes. In the adult, however, the two parts are still 

 close together, being separated by only the drum membrane, which is 

 covered on one side with ectoderm and on the other with entoderm. 



The ectodermal groove becomes surrounded by several nodular eleva- 

 tions of skin, which coalesce in a definite manner to make the projecting 

 auricle (pinna). Its depression deepens, becoming the external acoustic 

 meatus, which extends inward to the tympanic membrane. The entoder- 

 mal portion of the spiracular cleft becomes in the adult an elongated 

 outpocketing of the pharynx, known as the auditory tube (Eustachian 

 tube). As seen in the section Fig. 475, the tube is separated from the 

 bottom of the meatus by a very thin layer of mesenchyma, which is later 

 included in the drum membrane. 



In the mesenchyma behind the spiracular cleft, a chain of three small 

 bones (the malleus, incus, and stapes) develops; it extends from the 

 meatus to the vestibule. The bony wall of the vestibule is deficient 

 at the small oval area where the stapes reaches it, so that the chain of bones 

 comes directly in contact with the fibrous covering of the perilymph space. 

 This area of contact is the fenestra veslibuli (i.e., window of the vestibule). 

 When the chain of bones vibrates back and forth, the motion of the stapes 

 is transmitted through the fenestra vestibuli to the perilymph, and waves 

 may pass up the seal a vestibuli and down the scala tympani, stimulating 

 the nerves of hearing in the cochlear duct. The blind termination of 

 the scala tympani rests against the lateral wall of the vestibule, where also 

 the bone fails to develop; the round fenestra cochlea is thus produced. It's 

 fibrous membrane may yield somewhat to the perilymph waves, thus 

 relieving tension in the cochlea. 



In Fig. 475 the fragments of the chain of bones together with neigh- 

 boring nerves are imbedded in a mass of mesenchyma. In a later stage 

 the outer end of the auditory tube expands, filling all the space between 



