190 
GENERAL OMNITROLOGY. 
planes of any cubical figure. In birds these terms do not apply so well 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 nearly exhibited. The whole set is tilted over backward to some 
degree, so that the (a) ''anterior" (though still 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 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 no 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 skull (fig. 70, asc). It makes much 
more than a semicircle — rather a horse-shoe. The inferior vertical (h) 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 (h) ', 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 (6) and (c), like the twining inosculation of {a) and (&), is weU 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 (b) 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 menihranons 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.) 
All that precedes relates to the hony labyrinth, — the scrolled cavity of the periotic bone. 
The memhranous 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 memhranous cochlea, which afterward becomes shut off 
from the main sac. This shut-off cochlear part lies between the scala tympani below and the 
scala 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 flviid, the endolymph, 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 office — that of equilihration, 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 
