OSSEOUS LABYRINTH. 715 
membranous labyrinth (labyrinthus membranaceus). These channels are named the 
ductus cochlearis, utricle, saccule, and membranous semicircular canals; the wtricle 
and saccule are lodged within the bony vestibule. 
OSSEOUS LABYRINTH. 
Vestibule.—The vestibule forms the central portion of the osseous labyrinth, and 
communicates behind with the semicircular canals and in front with the cochlea. It 
is somewhat ovoid in shape, its long axis being directed forwards and outwards. It 
measures about 6 mm. antero-posteriorly, 4-5 mim. from roof to floor, and about 3 mm. 
from without inwards. Its outer wall is directed towards the tympanic cavity, and 
in it is seen the fenestra ovalis, which, in the recent state, is closed by the toot of 
the stapes. Its inner wall corresponds with the bottom of the internal auditory 
meatus and presents, at its antero-inferior part, a rounded depression, the recessus 
sphericus, which lodges the saccule. This recess is perforated by some twelve or 
fifteen small foramina, which constitute the macula cribrosa media and transmit the 
filaments of the auditory nerve for the supply of the saccule. The recessus 
spheericus is limited above and behind by an oblique ridge, the crista vestibuli, the 
anterior extremity of which is triangular in shape and named the pyramid (pyramis 
vestibuli). Posteriorly, this crista divides into two limbs, between which is a small 
depression, the recessus cochlearis of Reichert, perforated by some eight small fora- 
mina, which give passage to the nervous filaments for the supply of the posterior 
extremity of the ductus cochlearis. Above and behind the crista vestibuli, in the 
roof and inner wall of the vestibule, is an oval depression, the recessus ellipticus, 
which lodges the utricle. The pyramid and adjacent part of the recessus ellipticus 
are perforated by some 25-30 small apertures, which constitute the macula 
_eribrosa superior seu major. ‘The foramina in the pyramid transmit the nerves to 
the utricle; those in the recessus, the nerves to the ampullary ends of the superior 
and external semicircular canals. Behind and below the recessus ellipticus is a 
furrow, gradually deepening to form a canal, the aqueductus vestibuli, which passes 
backwards through the petrous bone and opens, as a slit-like fissure, about midway 
between the internal auditory meatus and the groove for the lateral sinus. This 
aqueduct measures 8-10 mm. in length and gives passage to the ductus endo- 
lymphaticus and a small vein. The “posterior part of the vestibule receives the 
five rounded apertures of the bony semicircular canals; its anterior part leads, by 
an elliptical opening, into the scala vestibuli of the cochlea. This opening 1s 
bounded inferiorly by a thin osseous lamella, the lamina spiralis ossea, which 
springs from the floor of the vestibule immediately to the outer side of the recessus 
spheericus, and forms, in tne cochlea, the bony part of the septum between the scala 
tympani below and the scala vestibul above. From the anterior part of the floor 
of the vestibule a narrow cleft, the fissura vestibuli, extends forwards into the bony 
canal of the cochlea. It is bounded internally by the lamina spiralis ossea, just 
referred to, and externally by a second, smaller lamina, the lamina spiralis 
secundaria, which projects inwards from the outer wall of the cochlea. These two 
lamina are continuous with each other around the posterior extremity of the 
fissure. 
Semicircular Canals (canales semicirculares ossei).—The semicircular canals 
(Figs. 521, 522), three in number, are situated above and behind the vestibule. 
They are distinguished from each other by their position, and are named superior, 
posterior, and external. They open into the vestibule by five apertures, since the 
inner extremity of the superior and the upper extremity of the posterior join to 
form a common canal or crus commune. Differing slightly i in length, each forms 
about two-thirds of a circle, one extremity of which is dilated and termed the 
ampulla (ampulla ossea). Somewhat compressed laterally, their greatest internal 
diameter is from 1 to 15 mm., whilst the diameter of the ampulla is about 2 mm. 
The superior semicircular canal (canalis semicircularis superior), 15-20 mm. in 
length, is vertical and placed transversely to the long axis of the petrous bone. 
Its convexity is directed upwards, and its position is indicated on the anterior sur- 
face of the petrous-temporal by an eminence. Its ampullated end (ampulla ossea 
