188 
GENERAL OBNITHOLOGY. 
nerve of hearing, remains in the hone, being expended upon the labyrinthine structures within 
— the vestibule J semicircular canals, and cochlea, which constitute the walls of the cavities in 
which the essential organ of hearing is snugly encased. 
If now, with a very fine saw — the saw^s now so much used for fancy scroll-work will 
answer the purpose — the whole periotic mass he cut away from the skuD, and then divided in 
any direction, the lahyrinth can be studied. It is best to make the section in some definite 
plane with reference to the axes of the whole skull, — the vertical longitudinal, or vertical 
transverse, or horizontal, — as the direction and relations of the contained structures are then 
more easily made out. Four or five parallel cuts will make as many thin flat slices of bone^ 
affording eight or ten surfaces for examination 5 the M^hole course of the labyrinthine cavity can 
be seen in sections which, when put together in the mind's eye, or held a little apart in their 
proper relations and visibly threaded with hristles, afford the required picture very nicely. It 
is extremely difficult to chisel out the affair from the bone in which it is embedded. At first 
glance the slices show a bewildering maze, — a continuous net- work or lattice-work of bone, in 
which the unaccustomed eye will recognize nothing but confusion. All this cancellated struc- 
ture, however, is pneumatic — the open-work tissue of the bone, containing air derived from 
the tympanic or eustachian cavities, and having nothing to do wdth the ear-passages proper. 
Parts of the hony lahyrinth will soon be recognized by their firm smooth walls and definite 
courses, as distinguished from the irregular interstices of the pneumatic bone-tissue. The bony 
labyrinth consists of an irregular central cavity, the vestibule; of a cavity, projecting like a 
beak downward and backward fi'om the vestibule, the cochlea; and of three horseshoe- shaped 
tubular cavities, above, behind, and below the vestibule, the semicircular canals, the ends of 
whose hollows all open into the vestibule. Imagine three hollow horseshoes, with their ends 
melted into a hollow inflation (vestibule), the opposite wall of which is a hoUow projection 
(cochlea) — or a hollow flat-iron (vestibule) with a long nose (cochlea) and three hollow handles 
(the canals). Or, see figs. 84 to 87, representing the contained membranous labyrinth, to which 
the containing bony labyrinth very closely conforms, as it is simply the bony caAdty whose walls 
encase the membranous and other soft structures. According as the sections have been made, 
numerous cross-cuts of the canals will be seen here and there as circular orifices ; the canals 
themselves lying curled like worms in the petrosal and occipital substance, their ends finally 
converging to the vestibular cavity. As compared with those of man, the parts are of great 
size ; in the eagle, the whole affair is as large as that part of one's thumb covered by the nail ; 
the whole length of the superior semicircular canal is an inch or more ; its calibre, I should 
judge, being absolutely about as great as in man. The cochlea, however, though not diminutive 
comparatively, is in a rudimentary condition as far as complexity of structure is concerned, in all 
Sauropsida, representing only the beginning of the cochlear structure of mammals. In the 
latter class, the cochlea is spirally coiled or whorled on itself like a snail- shell (whence the 
name — cochlea, a snail), making at least one turn- and a half, sometimes five (two and a half in 
man) ; with a centre-post or modiolus around which winds a bony flange, the lamina spiralis^ 
a membranous extension of M^hich to the cochlear out-wall divides the cavity into two com- 
partments or scalce (scala, a flight of stairs) ; it is just like a spiral stairway, only an inchned 
plane instead of a series of steps. The membranous extension of the bony spiral lamina to the 
side-wall obviously throws the cavity, as just said, into two spirals, which only intercommuni- 
cate at the top, where the modiolus ends in a funnel-shaped expansion, the infundibulum, 
beneath the apex of the snail- shell, the cupola. A marble rolling down the upper stairway 
would fall into the vestibular cavity; this division of the cochlea is therefore the scala vestibuli. 
The marble starting from the other side of the infundibulum would roll along the under stair- 
way, and if nothing stopped the way, would fall through the fenestra rotunda into the tym- 
panic cavity ; this is therefore the scala tympani. The first marble would also eventually 
reach the tympanum, through the vestibule, and out of the fenestra ovalis, if the foot of the 
