492 THE RABBIT CHAP. 



It will be remembered that in the frog the occipital region is ossified 

 by exoccipitals only, the parietals and frontals of either side are fused, 

 there are no ali- or orbito-sphenoids, the cartilaginous walls of the 

 anterior part of the cranium are ossified as a sphenethmoid, and that 

 the floor of the skull is supported by a membrane bone, the para- 

 sphenoid (pp. 40-44). 



The auditory capsules are comparatively loosely wedged 

 in laterally between the parietal and occipital segments ; in 

 the embryo each is ossified from three centres, instead of 

 one (the pro-otic) as in the frog, but these early unite to 

 form the periotic bone (peri\ as the ossified auditory capsule 

 is called. The internal or petrous portion of this bone 

 (Fig. 133) encloses the membranous labyrinth of the ear 

 and is very dense and hard ; posteriorly it is produced out- 

 wards as the porous mastoid portion, which is visible on the 

 outer side of the skull (Fig. 132 A). Closely applied to the 

 outer surface of each periotic is a bone called the tympanic* 

 consisting of a tubular portion ibove the edge of which sur- 

 rounds the auditory opening (aud. me) to which the cartilage 

 of the pinna is attached, and of a swollen portion, or tympanic 

 bulla (ty. bul) below : this encloses the tympanic cavity, and 

 in it, at the base of the tubular portion, is an incomplete 

 bony ring to which the tympanic membrane is attached 

 (Fig. 143). The tympanic is incomplete on its inner side, 

 where its cavity is closed by the outer wall of the periotic, 

 and between the two, at the antero-inferior angle of the 

 former, is the aperture by which the Eustachian tube 

 leaves the tympanic cavity (compare p. 45). When the 

 tympanic is removed, two small holes are seen on the outer 

 wall of the periotic : the anterior of them is \hefenestra ova/is* 

 and is plugged by the stapes which, together with two small 

 bones, the malleus and incus (Fig. 143), forms the chain of 

 auditory ossicles to be described later in connection with 



