426 SENSE-ORGANS AND INTEGUMENT. 



The fenestra vestibuli (Y) 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 e) 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 (d}> and posterior (e). The 

 external one (<:) 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, d] and pos- 

 terior (e} are in nearly dorsoventral planes, but at right angles 

 to each other. The superior (d) lies in the ridge forming the 

 caudal boundary of the appendicular fossa. The posterior (e) 

 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 (d) and posterior (e) canals join at one end and enter 

 the vestibule by a common opening. One end of the external 

 canal (^) 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 (<:) 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. 



