250 ON THE STRUCTURE OF THE SKULL. 



The outer lip of the bulla and the auditory meatus are anchylosed 

 with that region of the pro-otic which corresponds with the legmen 

 tympani in Man. The inner lip of the tympanic bulla is, as is the 

 case with the corresponding edge of the tympanic bone of Man, 

 applied against the opisthotic, but it does not anchylose with this 

 bone in the Beaver ; at any rate, for the greater part of its extent. 

 Consequently, a very narrow cleft or fissure, leading into the tym- 

 panum, is opened up, if the inner lip of the bulla is gently prized 

 away from the periotic mass in this region. I shall term this 

 the " tympano-periotic fissure." The great difference between the 

 tympanic bone of Man and that of the Beaver arises from the 

 circumstance, that, in Man, by far the greater part of the bone 

 is occupied by the external auditory meatus ; the interval be- 

 tween the groove for the attachment of the tympanic membrane 

 and the inner edge of the tympanic bone — which forms the floor 

 of the tympanum — being quite insignificant, except in the region 

 of the Eustachian tube. In the Beaver, on the other hand, this 

 part of the tympanic bone is greatly enlarged, and constitutes 

 more than the inner half of the bulla tympani. 



The tympanic bone and the periotic being thus anchylosed 

 together externally (though only coadjusted internally), form one 

 bone in the adult Beaver. But this " tympano-periotic bone " is 

 not anchylosed with any of the adjacent bones, even the squa- 

 mosal remaining perfectly distinct. Nor, indeed, is it fixed to 

 them by very firmly interlocking sutures, so that in the dry skull 

 it may be pushed out without difficulty. It is held in place, in 

 fact, only by the descending post-auditory process of the squa- 

 mosal (answering to the posterior part of the mar go tympanicus), 

 which curves behind the external auditory passage ; and by the 

 fitting in of the " pars mastoidea " between the ex-occipital and 

 supra-occipital. 



Of the vast multitude of modifications undergone by the 

 Mammalian skull, I select for comment, in this place, only a few 

 of the most important, such as, lstly, those which are the result 

 of unusual forms or combinations of bones in skulls not other- 

 wise abnormal. 2ndly. Those which are exhibited by the skulls 

 of the higher Mammals as compared with the lower, ordly. 



