IX.] OF THE DOG. 113 



incomplete above, surrounding the inferior three-fourths of 

 the membrana tympani, but it undergoes a considerable 

 development in the course of the first few mondis. The 

 external edge of the ring is produced horizontally outwards 

 to form the short, bony, external auditory meatus; while 

 the under and inner surface is greatly expanded, to form 

 the conspicuous rounded prominence, hollow within, called 

 the auditory bulla, which abuts against the outer edge of 

 the basioccipital below.' 



The space that is left among this group of bones, — bounded 

 by the periotic (the part in which the before-mentioned 

 fenestrse are situated) within, the periotic and squamosal 

 above, the tympanic and its bullate expansion below, behind, 

 and in front, and by the meatus auditorius externus, closed 

 in the natural state by the membrana tympani, to the outer 

 side, — is called the tympanic cavity. 



It contains within it the ossicula auditus, three small 

 bones called malieus, incus, and stapes, which, articulated 

 together, stretch across the cavity from the membrana 

 tympani to Xho. fenestra ovalis^ The cavity has an opening 

 at its antero-internal angle, through which the Eustachian 

 tube, connecting the tympanum with the pharynx, passes. 



The inner side of the bulla is perforated lengthwise by a 

 canal, which commences posteriorly within the margin of 



1 The whole of the bulla is generally considered as belonging to the 

 tympanic bone, but its inner part in many mammals is developed in a 

 distinct cartilaginous lamella, interposed between the lower edge of the 

 tympanic ring and the base of the skull. This may ossify from a sepa- 

 rate nucleus, or by extension of bony deposition inwards from the true 

 tympanic. The development of this region of the skull in the Mam- 

 malia still offers an interesthig field for investigation. 



2 As these bones are, in the Mammalia, completely subservient to 

 the organ of hearing, their modifications will not be described in the 

 present work. 



I 



