ANATOMY OF BIRDS 



275 



inner ear or proper acoustic cavity. There will be 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 ovalis ; it lies in the ob- 

 literated suture between the prootic 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 oi 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 proper. 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 " stirrup-bone," 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-post" 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 of fowl, about x' 



4 ; after Parker, st, its 

 foot, fitting fenestra ' 

 ovalis; fast, main shaft, 

 or mediostapedial ele- 

 ment; sst, supi-astape- 

 dial ; est, exti'astape- 

 dial ; ist, infrastape- 

 dial, its end repre- 

 senting a rudimentery 

 stylohyal ; /, a fenestra 

 in the extrastapedial. 

 (See St in situ. Fig. 71, 

 and its embryonic for- 

 mation. Fig. 67.) 



capsule of the ear ; but no trace of this relation 

 persists. Fig. 83 shows the mature stapes of a 

 fowl, and indicates its several elements which have 

 received special names. In skulls prepared with 

 sufficient care, the stapes may be seen in situ, as 

 in Fig. 71, st, — an extremely delicate rod, stepped 

 into the fenestra ovalis by its foot, the other end 

 protruding freely, and bearing in many cases its 

 hammer-like or claw-like stapedial elements. A 

 stapes I have just picked 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, s^. In a i-aven'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 



