94 AVES. 



The external auditory meatus is short, and completely formed by 

 bone posteriorly, where it forms a conchiform expansion. The mem- 

 brana tympani large and consisting of several lamellae, is directed ob- 

 liquely backward and inward, and differs in form from that of the 

 Mammalia, by being stretched so as to be convex or infundibuliform ex- 

 ternally toward the meatus, instead of internally, as in the human 

 subject. The tympanic cavity is spacious and irregular, and re- 

 ceives the orifices of the two mostly osseous Eustachian canals 

 which nearly coalesce together at their faucial extremity into a com- 

 mon opening. Other small apertures conduct from this cavity into 

 the osseous cells of the surrounding bones, which correspond to the 

 cells of the mastoid process, and occasionally extend over the whole 

 cranium. The walls of the tympanic cavity are not truly closed, but 

 communicate directly with all the cranial bones, and even with the os 

 quadratum. 



A veritable chain of auditory ossicles may be distinguished, one 

 of which is style-shaped and bony, while the two others remain in a 

 cartilaginous condition. The external of these cartilages, corre- 

 sponding to the malleus, is mostly of a triangular shape, perforate, 

 and provided frequently with a long process, abuts against the mem- 

 brana tympani. The second internal but smaller cartilage may be 

 frequently detached from the malleus, which it serves to unite with 

 the stapes or columella ; it may be regarded as the rudiment of the 

 incus. The principal bone is the long styliform analogue of the 

 stapes, the columella or bacillus ; its base expands into a broad oval 

 plate which is lodged in the foramen ovale, and through this the son- 

 orous impressions are transmitted to the aqueous fluid of the laby- 

 rinth. More frequently the columella is found to be broader inferi- 

 orly, and by being provided with two crura, to assume its character- 

 istic stirrup-like figure, as is the case in the Pelican, Raven, &c., 

 where it resembles the stapes of many of the Mammalia, e. g. the 

 Kangaroo. 



Only a single muscle can be detected for moving the auditory os- 

 sicles ; this would seem to correspond to the laxator rather than the 

 tensor tympani, the nominally implied action of the latter muscle 

 upon the membrana tympani being usually effected by the elasticity 

 of the malieal cartilage. The muscle in question arises from the 

 posterior part of the tympanic cavity, and is inserted into the malieal 

 cartilage, expanding also in a tendinous manner upon the membrana 

 tympani. 



The Labyrinth consists of very compact bony parietes, but ia 



