206 MAMMALIAN ANATOMY 



forms the fossa for the heads of the remaining ear-ossicles, the malleus 

 and incus. 



Behind these two features, firmly attached to the middle of the 

 curved posterior border of the tensor tympani fossa, is the inner end 

 of a bony bridge, which is a continuation inward, forward, and upward 

 of the end of the plate which forms the posterior upper wall of the 

 smaller fossa on the under surface of the squamous. When this bar 

 is broken away, just above it and behind its point of attachment to 

 the petrous is seen the slit-like opening of the aquseductus Fallopii. 

 This opening is continued downward and inward as a short, curved 

 groove, which in turn is continuous with the groove between the 

 petrous and the mastoid, and, as has been already described, is con- 

 verted into the extrapetrous portion of the aqua3ductus Fallopii by 

 the concave posterior inner surface of the bridge projecting from 

 the squamous, and by the groove between the posterior ends of the 

 ectotympanic and entotympanic. 



A narrow, slightly crescentic fossa lies behind and to the medial 

 side of the groove on the petrous, separated from it by a ridge which 

 contains the anterior end of the external semicircular canal. The 

 stapedius muscle is lodged in this fossa, and enters the tympanum 

 through the slit which is visible between the promontory and the thin 

 inner edge of the bony bridge from the squamous. This fossa is 

 practically the posterior limit of the inferior surface of the petrous. 

 Its long axis is directed from, within outward and forward. Its ante- 

 rior wall is convex, and is the posterior outer wall of the vestibule 

 contained in the petrous. Its posterior wall is concave in both di- 

 rections, and contains near its upper margin the greater part of the 

 external semicircular canal. 



The anterior border of the petrous (Fig. 138) separates the anterior 

 and inferior surfaces. It is directed from without inward and forward. 

 It consists of three equal parts. The middle part is directed slightly 

 more transversely to the long axis of the skull than the outer and 

 inner portions, from which it is separated on the outside by a hook, 

 on the inside by a groove leading to the hiatus Fallopii on the ante- 

 rior surface. It is rounded, forms the anterior margin of the fossa 

 for the tensor tympani on the inferior surface, and articulates with the 

 alispheuoid. The outer part of the anterior border is sharper, passes 

 into the basal borders of the anterior and inferior surfaces, and articu- 



