THE SKULL. 33 



the external auditory meatus is a nearly dorsoventral groove, 

 which, when the bone is articulated, forms a part of the boundary 

 of the stylorhastoid foramen (Fig. 22,7); just ventrad of this 

 groove is a pit (k] which lodges the tip of the tympanohyal 

 bone. 



Craniad the bone is produced into a short spine, the styli- 

 form process (^), which lies in a horizontal groove in the 

 ventral surface of the basisphenoid. Laterad of this spine is 

 a groove for the tuba auditiva or Eustachian tube. 



The medial surface (Fig. 24) presents in the middle near 

 its ventral margin a short triangular spine which lies in the 

 natural state against the ventral surface of 

 the basilar portion of the occipital. 



Caudad of this spine the surface is 

 marked by two or three vertical parallel 

 grooves (Fig. 23, ;;/). They indicate the 



portion of the bone which bounds the jugu- r 



c ... . . , FlG - 2 *- TYMPANIC 



lar foramen, and are possibly impressions of BULLA, ISOLATED, 



the ninth, tenth, and eleventh nerves. MEDIAL SURFACE. 



_, i . i r 1-1 a ' inner end of audi- 



The dorsal two-thirds of the medial tory meatus; b, partition 

 surface is lacking in the disarticulated bulk dividin tympanic cav. 



ity; c, styhiorm process. 



(Fig. 24), so that the cavity of the bone is 

 exposed. This opening is in the natural state closed by the 

 petrous bone. The caudal end is rough where it is overlaid 

 by the jugular process. , 



On the inner surface of the tympanic bulla is seen the thick- 

 ened margin of the inner end of the auditory meatus (Fig. 

 24, a). To it is attached the membrana tympana. In the 

 median dorsal line this margin is notched for the reception of 

 the incus and head of the malleus. From the lateral wall of 

 the cavity at the line of junction of the ectotympanic and ento- 

 tympanic a thin bony partition (^) rises. It runs almost directly 

 mediad ; is concave dorsally and divides the tympanic cavity 

 into two chambers. 



The Petrous Portion (Fig. 23, b, and Fig. 25). This con- 

 sists of two parts, a very dense part (the petrous portion proper, 

 Fig. 25), which has the form of a triangular pyramid and 

 encloses the auditory labyrinth, and a less dense part, the 



