188 GENERAL ORNITHOLOGY. 



nerve of hearing, remains in the bone, being expended upon the labyrinthine structures within 

 — the vestibule, 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 saws now so much used for fancy scroU-work will 

 answer the purpose — the whole periotic mass be cut away from the skull, and then divided in 

 any direction, the labyrinth can be studied. It is best to make the se(;tion in some definite 

 plane with reference to the axes of the whole skuU, — 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, 

 afibrding eight or ten surfaces for examination ; the Avhole 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 bristles, afltord the required picture very nicely. It 

 is extremely difficult to chisel out the aftair 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 unaccustoified 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 with the ear-passages jiroper. 

 Parts of the bony labyrinth 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 from the vestibule, the cochlea; and of three horseshoe-sha})ed 

 tubular cavities, above, behind, and belo\A' 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 hollow projection 

 (cochlea) — or a hollow flat-iron (vestibule) with a long nose (c( >chlea) 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 aff"air 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 conceraed, in all 

 Sauropsida, representing only the beginning of the cochlear structure of mannnals. 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 iu 

 man) ; with a centre-post or modiolus around which winds a bony flange, tlie lamina spiralis, 

 a membranous extension of which to the cochlear out- wall divides the cavity into two com- 

 partments or scala (scala, a flight of stairs) ; it is just like a spiral stairway, only an inclined 

 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 ttvo 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 



