32 THE SKELETON OF THE CAT. 
roughened portion of its inner surface overlies the margins of 
both these bones. 
The zygomatic process (d) is formed by the confluence of 
two roots. One of these starts from the ventral end of the. 
lambdoidal ridge and passes along the ventral margin of the 
squama dorsad of the external auditory meatus. The other 
arises abruptly from the cranioventral angle of the bone. The 
process thus formed is at first broad and passes horizontally 
laterad and slightly craniad. It soon grows more slender and 
turns gradually craniad, while at the same time it twists so that 
the surface which is dorsal at the base becomes medial at the 
tip; the posterior root which is continuous with the caudal 
border at the base is continuous with the dorsal border at the 
apex. On the ventral surface of the base is a transversely 
elongated concave articular surface, the mandibular fossa (f), 
for the condyloid process of the lower jaw. Caudad of this is 
a sharp transverse ridge, the postmandibular or postglenoid 
process (g), and craniad of the lateral end of the fossa a slight 
tubercle, the tuberculum articulare (Z). Near its apex the 
zygomatic process is more slender and its ventral border is 
bevelled for articulation with the malar or zygomatic bone. 
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 sub- 
stance is very compact. Unlike the tympanic of most other 
mammals it is developed from two bones, known as the ecto- 
tympanic (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, ¢) 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, 2), which leads into the tympanic cavity. Caudad of 
