HEAD AND NECK OF THE HORSE 



157 



tlie specimens the auricular cartilages must be removed, and the bone 

 should be cleaned as far as possible. 



In order to examine the cavity of the tympanum, the lower part of 

 the osseous bulla should be snipped away bit by bit. It will be observed 

 that the cavities of the osseous bulla and the tympanum communicate 

 freely vv^ith each other ; indeed, the cavity of the bulla must be regarded 

 as' a recess or extension of the tympanic cavity. 



The cavity of the tympanum (Cavum tympani). — The cavity 

 of the tympanum ^ is a narrow chamber within the temporal bone, 

 between the deep end of the external acoustic meatus and the internal 

 ear. Its lateral wall (paries membranacea) is formed almost entirely by 



External acoustic meatus. - 

 Epitympanic recess. 



Incus. 

 Malleus. 



Stapes. 



Cavity of tympanum. 



Membrana tympani 



secundaria. 



Membrana tympani. 



Entrance to aquaeductus 



cochlese. 



Osseous bulla. 



Auditory (Eustachian), 

 tube. 



.Semicircular canal. 

 -Semicircular duct. 



-Utricle. 



. .Vestibvile. 



..Utriculo-saccular duct. 

 -Endolymphatic duct. 

 ..Saccule. 

 -Ductus reuniens. 

 -Csecum vestibulare. 



Cascum cupulare. 



Helicotrema. 



Cochlear duct. 



Scala vestibuli. 



Scala tympani. 



Fig. 70. — Diagram of the tympanum and internal ear. 

 the thin, translucent membrana tympani. On the medial wall (paries 

 labyrinthica), which separates the cavity from the labyrinth of the 

 internal ear, there is a well-marked projection, the proviontory 

 (promontorium), formed by the first coil of the cochlea of the internal 

 ear, above and slightly in front of which is the fenestra" vestibuli 

 closed by the base of the stapes. Below the promontory is the fenestra 

 cochlece, occluded by the secondary tympanic membrane (membrana 

 tympani secundaria). Leading from the anterior and lower part of the 

 cavity is the narrow opening of the auditory tube of Eustachius, by 

 which air enters the tympanum. 



The roof (paries tegmentalis) of the tympanic cavity is crossed by 



1 Tympanum [L.], a drum. 



2 Pg,iiestra [L.], a window. 



