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 stylomastoid foramen (Fig. 22, 7); just ventrad of this 
groove is a pit (£) which lodges the tip of the tympanohyal 
bone. : 
Craniad the bone is produced into a short spine, the styli- 
form process (g), 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, mm). They indicate the 
portion of the bone which bounds the jugu- |, ca 
: IG, 24. — TYMPANIC 
lar foramen, and are possibly impressions of Butta, IsoLaTeD, 
the ninth, tenth, and eleventh nerves. MEDIAL SURFACE. — 
f ‘ a, inner end of audi- 
The dorsal two-thirds of the medial tory meatus; 4, partition 
surface is lacking in the disarticulated bulla dividing tympanic cav- 
ity; ¢, styliform 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 (6) rises. It runs almost directly 
mediad; is concave dorsally and divides the tympanic cavity 
into two chambers. 
The Petrous Portion (Fig. 23, 6, 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 
