190 GENEBAL OBKITHOLOGT. 



planes of any cubical figure. Ill birds these terms do not apply so weR to the situation of the 

 canals with reference to the axes of the body, nor to the direction of the loops; neither is 

 mutual perpendicularity so pearly exhibited. The whole set is tilted over backward to some 

 degree, so that the (o) " anterior" (though stUl superior) loops back beyond either of the others ; 

 the (6) "posterior" loops behind and below the (c) horizontal, which tilts down backward; 

 the verticality of the planes of (a) and (6) is better kept. The canals may be better known 

 as the (a) superior (vertical), and (6) inferior (vertical), and (c) internal (horizontal). What- 

 ever its inclination backward, there is uo mistaking (a), much the longest of the three, looping 

 high up over the rest, exceeding the petrosal and bedded in the occipital, the upper limb and 

 loop of the arch bas-relieved upon the inner surface of the skuU (fig. 70, use). It makes much 

 more than a semicircle — rather a horse-shoe. The inferior vertical (6) loops lowest of all, 

 though little if any, of it reaches further backward than the great loop of (a) ; it is the second in 

 size; in shape it is quite circular, — rather more than a half-circle. Its upper limb joins the 

 lower limb of (a), as in man, and the two open by one orifice in the vestibule ; but it is not 

 simple union, for the two limbs, before forming a common tube, twine half-round each other 

 (like two fingers of one hand crossed). The loop of (6) reaches very near the back of the skull 

 (outside). The canal (c) is the smallest, and, as it were, set within the loop of (6), though its 

 plane is nearly the opposite of the plane of (6) ; and the cavities of (6) and (c) intercommuni- 

 cate at or near the point of their greatest convexity, farthest from the vestibule. This decus- 

 sation of (5) and (c), like the twining inosculation of (a) and (6), is well known. It may not 

 be so generally understood that there is (in the eagle if not in birds generally) a third extra- 

 vestibular communication of the canals. My sections show this perfectly. The great loop of 

 (a), sweeping past the decussating-place of (6) and (c), is thrown into a cavity common to all 

 three. Bristles threaded either way through each of the three canals can all three be seen 

 in contact, crossing each other through this curious extra-vestibular chamber, which may be 

 named the trivia, or "three-way" place. (The arrangement I make out does not agree well 

 with the figure of the owl's labyrinth given by Owen, Anat. Vert, ii, 134. The trivia is at 

 the place where, in fig. 84 or 85, the three membrcmous canals cross one another. It does not 

 follow, however, that these contained membranous canals intercommunicate, and it appears 

 from Ibsen's figures that they do not. Study of these admirable illustrations, with the 

 explanations given under them, should make the details perfectly clear to the reader.) 



AU that precedes relates to the bony labyrinth, — the scrolled cavity of the periotic bone. 

 The membranous labyrinth is a sac lying loosely in the hollow of the bone, and shaped just like 

 it, lining the hollow of the vestibule and tubes of the semicircular canals. Withdrawn intact, 

 it would be a perfect " cast " of the labyrinth. Originally, this sac is also continuous with one 

 in the cavity of the cochlea, called the membranous cochlea, which afterward becomes shut oflf 

 from the main sac. This shut-oif cochlear part lies between the soala tympani below and the 

 soala vestibuli above ; its interior is the scala media. If demonstrable in birds, it must be quite 

 as rudimentary as the other scalse. The membrane is not attached to the bony walls of the 

 labyrinth, but is separated by a space containing fluid, the perilymph, which also occupies the 

 scala vestibuli and scala tympani. A similar fluid, the endoh/mph, is contained in the cavity of 

 the membranous labyrinth, and scala media of the cochlea ; in it are found concretions, or oto- 

 liths, of the same character as the great " ear-stones " so conspicuous in many fishes. This 

 lymph has a wonderful ofllce — that of equilibration, enabling the animal to preserve its 

 equilibrium. The labyrinth and its contained fluid may be likened to the glass tubes filled 

 with water and a bubble of air, by a combination of which a surveyor, for example, is enabled 

 to adjust his theodolite true to the horizontal. Somehow a bird knows how the fluid stands in 

 the self-registering levelling-tubes, and adjusts itself accordingly. Observations made on 

 pigeons show that " when the membranous canals are divided, very remarkable disturbances 

 of equilibrium ensue, which vary in character according to the seat of the lesion. When the 



