MEMBRANOUS LABYRINTH. TY 
modiolus like the thread of a screw, and, projecting about halfway into the cochlear 
tube, incompletely divides it into two passages, of which the upper is named the 
scala vestibuli; the lower, the scala tympani. The lamina spiralis ossea commences 
at the floor of the vestibule, near the fenestra rotunda, and ends close to the apex 
of the cochlea in a sickle-shaped process, the hamulus, which assists to form an 
aperture named the helicotrema. In the basil coil the upper surface of the spiral 
lamina forms almost a right angle with the modiolus, but the angle becomes more 
and more acute on ascending the tube. In the lower half of the basil coil a second 
smaller bony plate, the lamina spiralis secundaria, projects inwards from the outer 
wall of the cochlea towards the lamina spiralis ossea, without, however, reaching it. 
If viewed from the vestibule the slit-like fissura vestibuli, alre eady referred. to 
(p. 715), is seen between the two osseous spiral laminee. A membrane, the mem- 
brana basilaris, stretches from the free edge of the lamina spiralis ossea to the outer 
wall of the cochlea and completes the septum between the scala vestibuli and 
scala tympani, but the two scale communicate with each other through the 
opening of the helicotrema at the apex of the cochlea. The scala tympani begins 
at the fenestra rotunda, through which, in the macerated bone, it communicates 
with the tympanic cavity; in the recent condition the fenestra is closed by the 
secondary tympanic membrane (vide p. 708). At the commencement of the scala 
tympani a crest, termed the crista semilunaris, stretches from the attached margin 
of the lamina spiralis ossea towards the orifice of the fenestra rotunda. Close to this 
crest is seen the inner orifice of the aqueductus cochlee, a canal measuring from 10 to 
12 mm. in length and opening on the under aspect of the petrous bone internal to 
the fossa jugularis. Through it there is established a communication between the 
scala tympani and the subarachnoid space, and through it, also, a small vein passes 
to join the inferior petrosal sinus. The scala vestibuli, the higher of the two pas- 
sages, begins in the vestibule; its diameter in the basal coil is less than that of the 
scala tympani, but in the upper coils it exceeds that of the latter. 
From what has been stated, 1t will be evident that an injection thrown into the 
scala vestibuli will travel through the helicotrema, and then down the scala 
tympani as far as the secondary tympanic membrane, and will also pass along the 
aqueductus cochlee into the subarachnoid space. 
Internal Auditory Meatus.—It is convenient, at this stage, to study the 
fundus of the internal auditory meatus, which has been referred to as fc rming the 
inner wall of the vestibule and the base of the modiolus. It is divided by a trans- 
verse ridge, the crista transversa, into two parts—an upper or fossula superior and a 
lower or fossula inferior. The anterior part of the fossula superior is termed the 
area facialis and exhibits a single large opening, the commencement of the aque- 
duct of Fallopius, for the transmission of the facial nerve. Its posterior part is 
named the area vestibularis superior and is perforated by the nerves for utricle and 
ampullze of the superior and external semicircular canals. The anterior part of 
the fossula inferior is termed the area cochlee and consists of the canalis cen- 
tralis and the surrounding tractus spiralis foraminosus, for the passage of the nerves 
to the cochlea. Behind the area cochlez, and separated from it by a ridge, is the 
area vestibularis inferior, which transmits the nerves to the saccule, whilst at the 
posterior part of the fossula inferior is seen a single foramen, the foramen singulare, 
which gives passage to the nerves for the ampulla of the posterior semicircular ’ 
canal. 
MEMBRANOUS LABYRINTH. 
The membranous labyrinth (labyrinthus membranaceus) assumes, more or less 
closely (Fig. 524), the form of the bony labyrinth in which it is situated, but does” 
not nearly fill it. It contains a fluid termed endolymph, while the space between 
it and the bony labyrinth is occupied by a fluid termed perilymph. The mem- 
branous semicircular canals and the membranous canal of the cochlea follow the 
course of their bony tubes and lie along the inner aspect of their outer walls. The 
bony vestibule, on the other hand, contains two chief membranous structures, the 
utricle and saccule. The former receives the extremities of the membranous semi- 
cireular canals, whilst the latter communicates with the membranous canal of the 
