188 GENERAL OBNITHOLOGY. 



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

 — the vestibule, semicircula/r ccmals, 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 scroll-work will 

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

 any direction, the labyrinth can be studied. It is best to make the section 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, 

 affording eight or ten surfaces for examination ; the whole 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, 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 bewUdering maze, — a continuous net- work or lattice- work of bone, in 

 which the unaccustomed eye vdll recognize nothing but confusion. AU this ccmcellated 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 proper. 

 Parts of the hony 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 j 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 camals, 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 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 affair is as large as that part of one's thumb covered by the naU ; 

 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 

 Sawropsida, representing only the beginning of the cochlear stracture of mammals. In the 

 latter class, the cochlea is spirally coiled or whorled on itself like a snaU-shell (whence the 

 name — cochlea, a snaU), 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 com 

 partments or scalts (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 two spirals, which only intercommuni- 

 cate at the top, where the modiolus ends in a funnel-shaped expansion, the infimdibulwm, 

 beneath the apex of the snail-shell, the cMpofa. 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 tympami. The first marble would also eventually 

 reach the tympanum, through the vestibule, and out of the fenestra ovaUs, if the foot of the 



