ORGANS' OF THE SENSES 525 



The Internal Ear or Labyrinth. 



The internal ear, which is the essential portion of the 

 organ of hearing, consists of three parts : the vestibule, the 

 semicircular canals, and the cochlea : these are cavities im- 

 bedded in the petrous bone, communicating with the tym- 

 panic cavity on the one side, by the fenestne ovalis and ro-^ 

 tunda ; and on the other, with the internal auditory canal. 

 The dense bone immediately surrounding these cavities is 

 termed the osseous labyrinth, in contradistinction to the 

 membranous labyrinth contained within them. The descrip- 

 tion of the form, &c. of the osseous labyrinth belongs rather 

 to descriptive than to general or microscopic anatomy, and 

 therefore will not here be entered upon. 



The osseous labyrinth contains a fluid which has been 

 called the perilymph, from its surrounding, though in the 

 vestibule and semicircular canals only, a hollow membranous 

 apparatus the membranous labyrinth, which itself contains 

 a fluid, the endolymph. 



The following account of the structure of the spiral lamina ; 

 of the cochlea ; the cochlear muscle ; the cochlear nerves ; 

 the membranous labyrinth; the vestibular and auditory 

 nerves, is copied from the " Physiological Anatomy." 



Of the Structure of the Spiral Lamina of the Cochlea. 

 " We shall term the two surfaces of this lamina, tympanic and 

 vestibular, as they regard, respectively, the tympanic or ves- 

 tibular scala. The osseous portion of the spiral lamina ex- 

 tends more than half way from the modiolus towards the 

 outer wall, and is perforated, as already described, by a series 

 of plexiform canals, for the transmission of the cochlear 

 nerves ; these canals, taken as a whole, lie close to the lower 

 or tympanic surface, and open at or near the margin of this 

 zone. The vestibular surface of the osseous zone presents, in 

 about the outer fifth of its extent, a remarkable covering, 

 more resembling the texture of cartilage than anything else, 

 but having a peculiar arrangement, quite unlike any other 

 with which we are acquainted. Being uncertain respecting 

 the office of this structure, we shall term it the denticulate 



TT 2 



