SEC. IV ANATOMY OF BIRDS 275 



innei- ear or proper acoustic cavity. There \v\\\ Le observed, in the 

 side-wall of the tympanic cavity, two definite openings near the 

 Eustachian orifice. One of these, anterior and superior to the other, 

 larger usually, and oval, is the fenestra oralis; it lies in the ob- 

 literated suture between the proiltic and opisthotic bones ; and when 

 the membranous curtain which closes it in life is gone, you look 

 through this " oval window " into the vestibular cavity of the ear 

 proper. The lower, posterior, circular orifice is the fenestra rotunda ; 

 through which round window in the opisthotic bone you look into 

 the cochlear cavity of the ear jiroper. Fenestra ovalis and f. rotunda 

 are generally close together, — only divided by a little bridge of 

 bone, or a mere bony bar. To the circumference 

 of the fenestra ovalis is fitted the expanded oval 

 foot of the trumpet-shaped columella auris, — the 

 stapes, or " stirrujvbone," as it is called in mammals 

 (Fig. 83, st). This is an elegant little bone, which 

 establishes mechanical connection between the 

 membrane closing the fenestra ovalis and the 

 tympanic membrane, — something on the principle 

 of the " sounding-jDost " inside a violin. It is 

 shown magnified greatly in its embryonic condition 

 in Fig. 67, and there seems to be primitively and 

 morphologically the proximal connection of the 

 hyoid hone (by ceratohyal elements) with the bony stapes"offowi7aboutx' 

 capsule of the ear ; but no trace of this relation foot/*fit«n^^**fenestra 

 persists. Fig. 83 shows the mature stapes of a f'vaiis;ms«, main shaft, 



^ , 1 ■ ^^ . -, T 1 , 1-11 "'■ mediostapedial ele- 



lowl, and mdicates its several elements which have ment; sxt, suprastape- 

 received special names. In skulls prepared with \\\^\\ 'v'; "infrastapei 

 suflicient care, the stapes may be seen in situ, as '^'"-V ^^^ ''"'^ '''^p''^- 



' . .seiiting a rudimentary 



in Fig. 71, st, — an extremely delicate rod, stepped ?tyiohyai ;/, a fenestra 

 into the fenestra ovalis by its foot, the other end (Sees? in^sltu, Fig'! ni 

 protruding freely, and bearing in many cases its nuation,'!''!".^ e?.')''^ ^°^' 

 hammer-like or claw-like stapedial elements. A 

 stapes I have just jiicked out of an eagle's ear is a fourth 

 of an inch long, with a stout foot, but a stem as fine as a 

 thread of sewing silk, and at the tympanic end a still finer 

 hair-like process, half as long as the main stem, from which 

 it stands out at a right angle. The ossification is perfect, and there 

 appears to have been another similar process which has broken off 

 from the cross-like figure shown in Fig. 71, st. In a raven's skull 

 before me the stapes has fallen into the fenestra ovalis, and lies 

 there with its head sticking out. Though perfectly loose, I cannot 

 withdraw it intact, as the expanded foot fits the hole too closely to 

 pass through in any position I have succeeded in placing it. It 

 appears to be about as large as the eagle's. Close examination at a 



