496 HISTORY OF THE HUMAN BODY 



tympanic muscles have thus had a history as old as the parts 

 to which they are attached, and form here, together with 

 their associated nerves and ossicles, groups of parts which 

 have retained their original relationships through all their 

 migrations and changes of form and function. 



Another characteristic mammalian element, not directly 

 within the tympanic cavity but closely associated with it, is 

 the tympanic bone (os tympanicum). This, when in its full 

 development, forms a complete bony ring or frame about the 

 outer tympanic membrane, and often develops in addition a 

 concave osseous shell or tympanic bulla, which forms a conspic- 

 uous object at the base of the skull and aids in protecting the 

 delicate parts of the middle ear. Occasionally, too, the bone 

 extends outwards to -form an osseous wall for the external 

 meatus. This bone remains distinct throughout life in mono- 

 tremes, marsupials and a few others, but in the majority of 

 cases, as in Man, it fuses with the petrous elements and becomes 

 eventually lost in the complex designated as the " temporal 

 bone." The homologies of this bone are uncertain, although 

 some consider it the same as sauropsidan quadrato-jugal. It 

 can hardly be a new osseous element, but that it appears here 

 in a new role and is thus a new bone physiologically is evident. 



The external ear, characteristic of the mammalia, is mainly 

 a cartilaginous structure covered by integument, and consists 

 of a round tube, the external auditory meatus, and an external 

 flap, the auricula [pinna]. The first of these, the meatus, 

 allows the outer tympanic membrane to sink below the surface 

 and still retain connection with the exterior and its curve 

 affords the membrane a more or less complete concealment. 

 In cases where the tympanic bone furnishes a prolonged tube 

 for this purpose, the external cartilaginous element is less ex- 

 tensive, and the two together form the wall of the canal. 



The auricula shows a large degree of adaptation, being very 

 large and mobile in cases where acute hearing is desired, for 

 example, in bats, and in most ungulates; and is reduced or 

 entirely wanting in many burrowing or aquatic forms. The 

 characteristic anthropoid ear is shaped at the base much as in 



