ORGAN OF HEARING IN MAMMALIA. 227 



extremity of a long and narrow meatus auditorius externus, which 

 is strengthened by a cartilaginous incomplete cylinder, protected 

 by a valve, but not provided with an external conch. This is 

 equally wanting in the Echidna, in which the external aperture 

 of the auditory canal presents the form of a vertical slit, shaped 

 like the italic f, one inch and a half in length : the margins of 

 the slit are tumid, and support a row of bristles which protect and 

 cover the orifice when recumbent. The meatus is remarkably 

 long ; the tube is strengthened in this Monotreme by a series of 

 incomplete cartilaginous hoops, connected together by a narrow 

 longitudinal cartilaginous band, so that its structure closely re- 

 sembles that of a trachea, fig. 301, a, a. The tympanic fossa is 

 almost entirely encircled with a slender hoop of bone, vol. ii. 

 fig. 197, 28, consisting of the anchylosed tympanic bone and mal- 

 leus. The portion which represents the tympanic bone, ib, a, and 

 which can be separated from the malleus in the young subject, is 

 a slender osseous filament bent into three-fourths of a circle, and 

 placed upon the inner margin of the tympanic fossa, its concavity 

 looking outward : this concavity is impressed with a fine groove 

 for the insertion of the membrana tympani : the posterior part of 

 the hoop passes across the commencement of the Eustachian 

 canal, and terminates in a free point upon the posterior wall of 

 the tympanic fossa : the anterior end of the hoop is applied to and 

 usually anchylosed with the longitudinal bar of the malleus. 



Only a small portion of this ossicle is contained within the 

 cavity of the tympanum ; the principal portion, ib. o, forms the 

 external and part of the posterior boundary of the bony meatus 

 auditorius, and is then continued forward in the form of a slender 

 pointed process ; the bone slightly expands as it extends back- 

 ward, and its broadest part is abruptly bent inward until it nearly 

 meets the posterior end of the tympanic hoop. From the extre- 

 mity of this inflected portion a slender compressed process, c, 

 extends to the centre of the space encircled by the bony hoop ; 

 it is attached by its whole length to the membrana tympani, and 

 represents the handle of the malleus. At the posterior margin 

 of the broad incurved part of the malleus there are two minute 

 ossifications in an incudial cartilage : the short and simple co- 

 lumelliform stapes, ib. d, ascends vertically from the innermost of 

 these tubercles, with the upper surface of which it is articulated ; 

 its opposite extremity closes the foramen ovale in the form of an 

 expanded plate. The membrana tympani is concave outwardly 

 at its middle part. 



In Marsupialia the chief instruction from the ear-organ is 



q 2 



