32 THE SKELETON OF THE CAT. 



roughened portion of its inner surface overlies the marg'ins 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 (/), 

 for the condyloid process of the lower jaw. Caudad of this is 

 a sharp transverse ridge, the postmandibular or postglenoid 

 process {g), tind craniad of the lateral end of the fossa a slight 

 tubercle, the tuberculum articulare (/-). 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, ^; 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 (h'igs. 22 and 

 2 2), 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 e.xternal 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, /), which leads into the tympanic cavity. Caudad of 



