214 



MAMMALIAN ANATOMY 



margin of the opening of communication between the two chambers. 

 A cleft is left between the promontory and the septum through which 

 vessels and nerves from the inner side of the bulla reach the tym- 

 panum. The septum is concave without and convex within, thus 

 encroaching on the inner chamber. Its upper free margin is covered 

 in front by the inner part of the front of the entotympanic part of 

 the superior surface of the bulla. It curves downward and outward 

 into the posterior edge of the anterior part of the ectotympanic supe- 

 rior surface and forms part of the floor of the Eustachian canal. It 



FIG. 146. 



Fossa for Malleus and Incus. 



Mastoid. 



Bridge, with Groove leading dou'n 

 to Stylo-mastoid Foramen. 



Foramen for Chorda Tympani. 

 Pit for Tympano-hyal. 



Buttar Septum. 



Groove for Vessels. 



MI - Entocranial Surface of Squa- 

 mous. 



Hook on Ectotympanic. 

 External Auditory Meatus. 



Eustachian Groove. 



Spine, completes Basisphenoidal 



Notch. 

 Spine, with Basisphfnoid. 



LEFT TEMPORAL BONE. INNER SURFACE. PETROUS PORTION REMOVED. 



ends behind by bifurcating (Fig. 14(3) ; the outer horn joins the 

 inner rim of the external auditory meatus, and the inner horn curves 

 inward and upward to become continuous with the posterior part of 

 the free outer margin of the superior surface of the bulla. The small 

 angle included between these horns forms the roof of the pit for the 

 tympano-hyal, and is sometimes perforated at the bottom. On the 

 outer horn, just above this angle is attached a small bony and carti- 

 laginous rod which stretches across the chamber and sometimes touches 

 the promontory between the fenestra rotunda and the fenestra ovalis. 

 This rod supports the chorda tympani nerve which enters the bulla 

 through a foramen just inside its base. The outer opening of the 

 foramen is seen in the front part of the terminal portion of the 

 aquseductus Fallopii. 



The outer chamber of the bulla forms the floor of the tympanum, 

 or middle ear. It is crescent-shaped, wider in front than it is behind. 



