The =tympanic= (Figs. 22 and 23, _c_; Fig. 24) is expanded into a large hollow olive-shaped bone which is known as the =auditory bulla= and encloses the tympanic cavity. Its substance is very compact. Unlike the tympanic of most other mammals it is developed from two bones, known as the =ectotympanic= (Fig. 22, _c′_) and =entotympanic= (_c_). These are strongly marked in young kittens, and can usually be easily distinguished in adult cats. The entotympanic (Figs. 22 and 23, _c_) forms the larger part of the bulla, constituting its ventral and medial surfaces; it is thin, smooth, and transparent. The ectotympanic (Fig. 22, _c′_) surrounds the external auditory meatus: it is thicker and more opaque than the entotympanic. The bulla lies ventrad of the squamous, and in an external view conceals a large part of the petrous.
On its lateral surface it presents near the dorsal border an irregular oval opening, that of the =external auditory meatus= (Fig. 22, _i_), which leads into the tympanic cavity. Caudad of 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, _j_); 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 =styliform= process (_q_), 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.
[Illustration: FIG. 24.--TYMPANIC BULLA, ISOLATED, MEDIAL SURFACE.
_a_, inner end of auditory meatus; _b_, partition dividing tympanic cavity; _c_, styliform process.]
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, _m_). They indicate the portion of the bone which bounds the jugular foramen, and are possibly impressions of the ninth, tenth, and eleventh nerves.
The dorsal two-thirds of the medial surface is lacking in the disarticulated bulla (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.