CHAP, in.] SKELETON OF THE HEAD AND TRUNK. 67 



by a foramen just behind the post-glenoid process a post-glenoid 

 foramen. This, 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 divided within into 

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



Fig. 3(3. VERTICAL SECTION OF THE AUDITORY BULLA OF THE TIGER (Flower). 



am. Meatus auditor-ins externus. 



BO. Basi-occipital. 



<>. Eustachian canal. 



ic. Inner chamber of bulla. 



oc. Outer chamber of bulla. 



l>t. Promontory of petrosal. 



s. The septum. 

 Sq. Squamosal. 

 t. Tympanic ring. 



'- The aperture of communication between the 

 two chambers. 



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

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

 wall a horseshoe-shaped prominence 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 incus ; 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 ossicula audit-As 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, 



