ix.] OF THE DOG. 133 



incomplete above, surrounding the inferior three-fourths of 

 the membrana tympani, but it undergoes a considerable 

 development in the course of the first few months. 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. 1 



The space that is left among this group of bones 

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

 mentioned fenestne 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 carity. 



It contains within it the ossicula auditus, three small bones 

 called malleus, incus, and stapes, which articulated together, 

 stretch across the cavity from the membrana tympani to the 

 fenestra tn-alis.- The cavity has an opening at its antero- 



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 interesting field for investigation, 



* As these bones are, in the Mammalia, completely sul>servient to 

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

 present work. A very detailed and fully illustrated account of them 

 has been given by Alban Doran in the Transactions of the Linnean 

 S<>ciety t Second Series, vol. i. " Zoology," 1879. 



Much has been written on the still unsettled question of the develop- 

 ment and the homologies of the auditory ossicles by ReUhert, Peters, 

 Huxley, Parker, Koelliker and Gegenbaur ; see also IV. Saltmky, 



