CHAP. III.] SKELETOX OF, THE HEAD AXD TRUNK. 



07 



by a foramen just bcliind the post-gleiioid process — a post-gJenoid 

 foramen. Tliis, however, is generally absent. 



The margin of the petrosal, above the cerebellar fossa and 

 internal meatus, developes a bony ridge, which unites with the 

 tentorial plate, before described as passing downwards and forwards 

 from within the parietal. 



The cavity of the bulla is almost completely di\aded within into 

 two very unequal parts by a bony septum which ascends from the 



Fig. 3G. — Vlktical Section of the Alditorv Bulla of the Tiger (Fhaxr). 



<im. Meatus aucUtorius externus. 



130. Basi-occipital. 



e. Eustachian canal. 



ic. Inner chamber of bulla. 



(»c. Outer clianiber of bulla. 



pt. Promontoiy of petrusal. 



s. The seiituin. 

 Sq. Sc(uaiii<iSiil. 

 t. Tympanic ring. 



■ The aperture of communication between the 

 two chambers. 



floor of the bidla. The outer and anterior chamber is much the 

 smaller. It is the true tympanic chamber, and has on its outer 

 wall a horseshoe-shaped prommence and groove — the tympanic ring 

 — to which the tympanic membrane (or membrane of the drum of 

 the ear) is attached. It also contains three very small and irregularly- 

 shaped bones : (1) the malleus ,- (2) the incia^ ; and (3), the stapes, 

 which stretch across from the inside of the tympanic membrane to 

 the opposite wall of the petrosal, and are known as the auditory 

 ossicles — ossicuia auditus — and will be described together with the 

 organ of hearing. A long process (munubrium) of the malleus is 

 conspicuous (see Fig. 35, A), passing downwards and forwards from 

 the upper part of the tympanic cavity. Also the Eustachian tube (e) 

 opens into this same outer chamber, superiorly and anteriorly, while 

 towards its hinder margin are two holes placed one above another, 



