SEC. IV ANATOMY OF BIRDS 279 



glance the slices show a bewildering maze, — a continuous network 

 or lattice-work of bone, in which the unaccustomed eye will recog- 

 nise nothing but confusion. All this cancellated structure, however, 

 is pneumatic — the open-work tissue of the bone, containing air 

 derived from the tympanic or Eustachian cavities, and having nothing 

 to do with the ear-passages proper. Parts of the hony labyrinth 

 will soon be recognised by their firm smooth walls and definite 

 courses, as distinguished from the irregular interstices of the pneu- 

 matic bone -tissue. The bony labyrinth consists of an irregular 

 central cavity, the vestibule; of a cavity, projecting like a beak 

 downward and backward from 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 horse-shoes, with their ends 

 melted into a hollow inflation (vestibule), the opposite wall of which 

 is a hollow 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 cavity 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 afiair 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 struc- 

 ture 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 which 

 to the cochlear out-wall divides the cavity into two compartments 

 or scalcB (scala, a flight of stairs) ; it is just like a spiral stairway, 

 only an inclined plane instead of a series of steps. The membran- 

 ous extension of the bony spiral lamina to the side-wall obviously 

 throws the cavity, as just said, into two spirals, which only inter- 

 communicate 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 



