426 SENSE-ORGANS AND INTEGUMENT. 
The fenestra vestibuli (2) is an opening in its bony wall, filled 
in the recent state by the stapes. On the medial surface of the 
vestibule are a number of minute orifices through which the 
auditory nerve enters. 
The semicircular canals (c, d, and ¢) are three curved 
tubes, each forming about two-thirds of a circle, and opening 
at both ends into the vestibule. They are imbedded in the 
petrous bone. The planes of the three tubes intersect nearly 
at right angles; from their position in man they have been 
named external (c), superior (@), and posterior (¢). The 
external one (c) is nearly horizontal in position, and surrounds 
the curved fossa (Fig. 25, /) for the stapedius muscle, caudad 
of the fenestra vestibuli. The superior (Fig. 173, @) and pos- 
terior (¢) are in nearly dorsoventral planes, but at right angles 
to each other. The superior (2) lies in the ridge forming the 
caudal boundary of the appendicular fossa. The posterior (¢) 
is situated in a continuation caudad of the ridge which forms 
the dorsal boundary of the internal auditory meatus. One end 
of each canal is slightly enlarged to form an ampulla. The 
superior (Z2) and posterior (¢) canals join at one end and enter 
the vestibule by a common opening. One end of the external 
canal (c) opens into the vestibule so close to one end of the 
posterior canal (e) that they appear to have a common opening, 
and this is commonly stated to be the case; according to 
Jayne, however, the openings are really separate. The two 
remaining ends (of the superior (@) and external (c) canals) also 
enter the vestibule separately, so that the three canals enter 
the vestibule by five openings. 
III. THE OLFACTORY ORGAN. ORGANON OLFACTUS. 
The olfactory organ consists of the olfactory mucous mem- 
brane, in the dorsocaudal part of the nasal cavity. The nasal 
cavity has been described in connection with the respiratory 
system. 
