382 HISTOLOGY. 



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 

 called the vestibule. Into it the semicircular and cochlear canals open. 



The middle and the 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. 188, p. 166) and an ectodermal 

 depression (Fig. 187, sp.), which meet one another. The latter becomes 

 surrounded by several nodular elevations which coalesce in a definite 



Ganglion 

 spirale. 



Modiolus. 



Scala vestibuli. 



Scala tympani. 



Ramus 

 vestibularis 



of the 

 nervus 

 acusticus. 



Meatus acusticus internus. 



FIG. 432. HORIZONTAL SECTION OF THE COCHLEA OF A KITTEN. X 8. 



The winding ductus cochlearis, x, crossed the plane of section five times. Above it in every case is the 

 scala vestibuli, and below it is the scala tympani. 



manner to make the projecting auricle [pinna] of the external ear. Its 

 depression deepens, becoming the external auditory meatus, the ectoderm 

 at the bottom of which passes over the tympanic membrane, thus forming 

 its outer layer. The entodermal 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. 433, the tube is sepa- 

 rated from the bottom of the meatus by a very thin layer of mesenchyma. 

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

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

 of 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. 



