THE AUDITORY APPARATUS. 



953 



promontory, and terminating at the mastoid foramen. It contains the facial 

 nerve which passes through the tympanic cavity.) 



3. The Mastoid Cells (Fig. 515, G). 



These cells occupy all the circumference of the tympanic cavity, except above. 

 They are small, more or less irregular, deep spaces, separated by thin partitions 



Fig. 515. 



RIGHT TYMPANIC CAVITY OF THE HORSE'S EAR (ANTERIOR PLANE, VERTICAL AND 

 TRANSVERSE SECTION). 



At Auditory canal; B, membrana tympani; C, malleus; D, incus; E, os orbiculare; F, stapes; G, 

 mastoid cells ; H, fenestra ovalis ; I, vestibule ; /, K, L, outline of the semicircular canals ; i/, 

 cochlea ; N, commencement of the tympanic scala. 



radiating around the tympanic circle, their free margin being turned towards the 

 centre of the cavity. 



In several animals, and particularly the Carnivora, the mastoid cells form a 

 special compartment in the tympanic case, communicating with the latter by 

 a single opening. The largest are situated at the inferior part of the tympanic 

 oavity, below the vaginal process of the temporal bone. (In the Sheq) and 

 -Goat, the mastoid cells and their bony septa are entirely absent.) 



4. The Bones of the Middle Ear (Fig. 516). 

 Four articulating bones (the ossicula auditus), named the malleus, incus, os 



