74 INNERVATION. [CHAP. xvni. 



and their cavity is not cylindrical, but slightly compressed on 

 the sides, and about a twentieth of an inch in diameter. Each 

 is dilated at one end into an ampulla, of more than twice the 

 diameter of the tube, and at the opposite end it opens out slightly 

 on entering the vestibule. Each canal lies in a different plane, the 

 direction of which being constant, should be carefully noticed in 

 relation to their function. The superior vertical canal is also an- 

 terior, and lies across the petrous bone. It forms about two-thirds 

 of a circle, and its extremities are more divergent than those of the 

 others. In the foetus the concavity of this canal is free, owing to 

 a deficiency in the substance of the petrous bone, and its arch 

 forms a projection within the cranium, even in the adult. The 

 ampulla is on its outer extremity. The inferior vertical canal is 

 also posteriory and runs parallel to the posterior surface of the 

 petrous bone, and therefore at right angles to the former. The 

 ampulla is at its lower extremity, and its upper end joins the 

 inner end of the former canal, to constitute a common canal an 

 eighth of an inch long, rather wider than those which join to form 

 it, and opening behind and below. The horizontal canal is also 

 inferior, and shorter than either of the others ; its arch is directed 

 outwards and backwards ; its ampullar extremity is close to that of 

 the superior vertical canal. 



3. The cochlea is, in shape, very like a common snail-shell. It lies 

 almost horizontally, its apex forwards and outwards, its base marked 

 near the bottom of the internal meatus, by a depression exhibiting 

 a spiral arrangement of pores for the reception of the cochlear divi- 

 sion of the auditory nerve. From base to apex extends the irregu- 

 larly conical axis, modiolus, or columella, which is perforated by 

 numerous branching channels, ascending from the pores just men- 

 tioned, and distributing the nervous filaments in regular succession 

 within the spiral cochlear canal which winds around the axis. This 

 spiral canal is about an inch and a half in length, if measured along 

 its outer wall, and diminishes gradually in size from the base to the 

 summit of the cochlea, where it ends in a cul de sac. At its com- 

 mencement it is about one-tenth of an inch in diameter, but at its 

 termination scarcely half that size. At its base it diverges somewhat 

 from the modiolus, to wards the tympanum and vestibule, and presents 

 three openings. Of these, one, free and oval, enters the vestibule ; 

 another is the fenestra rotunda, communicating with the tympanum 

 in the dry bone, but filled up in the recent state by a proper mem- 

 brane, the membrana tympani secundaria ; the third is the minute 

 orifice of the aqueductus cochlece, a funnel-shaped canal leading to 



