716 THE ORGANS OF SENS 
superior) is anterior and external, and opens into the vestibule immediately above 
that of the external canal. Its opposite extremity joins the non-ampullated end 
of the posterior canal to form the crus commune, which is about 4 mm. in length 
and opens into the upper and inner part of the vestibule. The posterior semicircular 
canal (canalis semicircularis posterior) is the longest of the three and measures from 
18-22 mm. Its ampullary end (ampulla ossea posterior) is placed inferiorly and 
opens into the lower and back part of the vestibule, where may be seen some 
six or eight small apertures, forming the macula cribrosa inferior, for the trans- 
mission of the nerves to thisampulla. Its upper extremity ends in the crus commune. 
The external canal (canalis semicircularis lateralis) is the shortest. It measures 
from 12-15 mm. and arches nearly horizontally outwards. Of its two extremities 
the external is ampullated (ampulla ossea lateralis), and opens into the vestibule 
immediately above the fenestra ovalis and in close relationship to the ampullary 
end of the superior canal. 
Although the three canals are generally regarded as opening into the vestibule by five orifices, 
some observers incline to the view that the ampullary ends of the superior and external canals 
form a common orifice, and that, consequently, there are only four openings for the three canals. 
Cochlea.'—When freed from its surroundings the cochlea assumes the form of 
a short cone (Fig. 525); the central part of its base (basis cochlez) corresponds 
Cupola 
—Hamulius 
Canalis centralis 
Canalis spiralis modioli 
— Modiolus 
Section through promontory ‘ 
Lamina spiralis 2 
ossea pe” / 
Fissura vestibuli__—/- 
~—Scala vestibuli 
: Lamina spiralis ossea 
Lamina spiralis ossea —— ‘ 2 
Scala tympani 
Tractus spiralis 
foraminosus 
=>__ 
Be \ 
Recessus cochlearis of vestibule Internal auditory meatus 
Fic. 523.—SECTION OF Bony COCHLEA. 
with the bottom of the internal auditory meatus, whilst its apex or ‘cupola 
is directed forwards and outwards, and comes into close relation with the canal for 
the tensor tympani muscle. It measures about 9 mm. across the base and about 
5 mim. from base to apex, and consists of a spirally-arranged tube, which forms from 
2} to 27 coils around a central pillar termed the modiolus. The length of the tube is 
from 28 to 30 mm., and its diameter, near the base of the cochlea, 2 mm. _ Its coils 
are distinguished by the terms basal, central, and apical; the first, or basal coil, 
gives rise to the promontory on the inner wall of the tympanum. 
The modiolus is about 3 mm. in height and diminishes rapidly in diameter from 
base to apex, while its tapered extremity fails to reach the cupola by a distance of 
1 mm. Its base (basis modioli) corresponds with the area cochlee on the fundus 
of the internal auditory meatus and exhibits the tractus spiralis foraminosus, which 
transmits the nerves for the basal and central coils of the cochlea and the foramen 
centrale, which gives passage to the nerves for the apical coil. The foramina of the 
tractus spiralis foraminosus traverse the modiolus, at first parallel to its long axis, 
but, after a varying distance, they bend outwards to reach the attached edge of the 
lamina spiralis ossea, where they become expanded and form by their apposition a 
spiral canal, the canalis spiralis modioli of Rosenthal, which lodges the ganglion of 
Corti, or ganglion spirale cochleez. [rom this spiral canal numerous small foramina, 
for the transmission of vessels and nerves, pass outwards to the free edge of the 
5 
lamina spiralis. The lamina spiralis ossea, a thin, flat shelf of bone, winds round the 
' In the following description the cochlea is supposed to be placed on its base. 
