210 MAMMALIAN ANATOMY 



from the base of the petrous, articulating by its irregular emarginate 

 anterior border with the lower part of the posterior border of the 

 parietal, and by the short rounded upper border with the occipital. 

 The posterior border is also the inner border of the posterior surface 

 of the mastoid. This portion is concave in both directions and 

 supports the lateral sinus. 



(2) Below and in front of this portion is a small irregular area 

 whereby the petrous completely coalesces with the mastoid (Fig. 132) ; 

 an attempt to break the two portions apart usually produces a fracture 

 through the mastoid and opens up the mastoid cells in the cancellous 

 tissue. 



(3) The third or lower portion of the inner surface of the 

 mastoid lies below the preceding ; it faces inward, downward, and 

 forward, and is applied to the outer posterior part of the entotympanic 

 of the bulla (Fig. 137). It is triangular in form, smooth, and concave; 

 the lower, rounded angle is directed forward and downward, and 

 is the end of the mastoid process proper. A sharp ridge passing 

 upward and backward from this angle separates this surface in front 

 from the groove with the same direction which lies between the 

 mastoid, the upper end of the ectotympanic, and the lower posterior 

 part of the squamous, and, when completed by the groove on the bulla, 

 forms the terminal portion of the aqiueductus Fallopii. The upper 

 anterior angle is truncated, and meets the posterior part of the basal 

 portion of the inferior surface of the petrous. The upper border is 

 also the lower border of the posterior surface ; the posterior border 

 is common to the external surface. 



THE AUDITORY BULLA. 



The auditory bulla is an oblong sac, whereof the upper, free rim, or 

 mouth, is applied to the base of the skull, covering in completely the 

 petrous and the adjoining edges of the squamous and mastoid portions 

 and the edges of the exoccipital, basioccipital, basisphenoid, and ali- 

 sphenoid. It is not firmly united to the skull, and is readily broken 

 free from its attachments with the sphenoid and squamous in front 

 and with the occipital and squamous behind. 



When removed from the skull it is seen to be composed of two 

 parts, an inner, thin-walled, entotympanic part, and an outer, denser, 

 ectotympanic part. Of these the entotympanic part (Fig. 143) is 



