492 THE RABBIT CHAP. 



It will be remembered that in the frog the occipital region 

 is ossified by exoccipitals only, the parietals and irontals 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 an investing bone, the parasphenoid (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 outwards 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 above the edge of which surrounds 

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

 the pinna is attached, and of a swollen portion, or tym- 

 panic 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 mem- 

 brane is attached (Fig. 143). The tympanic is incom- 

 plete 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 the fenestra ovalis and is plugged 

 by the stapes which, together with two small bones, 

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

 auditorv ossicles to be described later in connection 



