MEMBRANOUS LABYRINTH. 465 



labyrinth! (liquor Cotunnii), and sending a reflection inwards upon the 

 nerves distributed to the membranous labyrinth. In the cochlea the mem- 

 brane of the labyrinth invests the two surfaces of the bony lamina spiralis, 

 and being continued from its border across the diameter of the canal to its 

 outer wall, forms the membranous lamina spiralis, and completes the se- 

 paration between the scala tympani and scala vestibuli. The fenestra 

 ovalis and fenestra r6tunda are closed by an extension of this membrane 

 across them, assisted by the membrane of the tympanum and a proper in- 

 termediate layer. Besides lining the interior of the osseous cavity, the 

 membrane of the labyrinth sends two delicate processes along the aque- 

 ducts of the vestibule and cochlea to the internal surface of the dura mater, 

 with which they are continuous. These processes are the remains of a 

 communication originally subsisting between the dura mater and the cavity 

 of the labyrinth.* 



The MEMBRANOUS LABYBINTH is smaller in size, but a perfect counter- 

 part with respect to form, of the vestibule and semicircular canals. It 

 consists of a small elongated sac, sacculus communis (utriculus communis) ; 

 of three semicircular membranous canals, which correspond with the os- 

 seous canals, and communicate with the sacculus communis ; and of a 

 small round sac (sacculus proprius), which occupies the anterior ventricle 

 of the vestibule, and lies in close contact with the external surface of the 

 sacculus communis. The membranous semicircular canals are two-thirds 

 smaller in diameter than the osseous canals. 



The membranous labyrinth is retained in its position by means of the 

 numerous nervous filaments which are distributed to it from the openings 

 in the inner wall of the vestibule, and is separated from the lining mem- 

 brane of the labyrinth by the aqua labyrinthi. In structure it is composed 



receiving a nervous fasciculus from the inferior branch of the vestibular nerve. 8. The 

 oblique semicircular canal with its membranous canal. 9. The common canal, resulting 

 from the union of the perpendicular with the oblique semicircular canal. 10. The mem- 

 branous common canal terminating in the sacculus communis. 11. The otoconite of the 

 sacculus communis seen through the membranous parietes of that sac. A nervous fasci- 

 culus from the inferior branch of the vestibular nerve is seen to be distributed to the 

 sacculus communis near the otoconite. The extremity of the sacculus above the otoconite 

 is lodged in the superior ventricle of the vestibule, and that below it in the inferior ven- 

 tricle. 12. The sacculus proprius situated in the anterior ventricle ; its otoconite is seen 

 through its membranous parietes, and a nervous fasciculus derived from the middle 

 branch of the vestibular nerve is distributed to it. The spaces around the membranous 

 labyrinth are occupied by the aqua labyrinthi. 13. The first turn of the cochlea; the 

 figure points to the scala tympani. 14. The extremity of the scala tympani correspond- 

 ing with the fenestra rotunda. 15. The lamina spiralis; the figure is situated in the 

 scala vestibuli. 16. The opening of the scala vestibuli into the vestibule. 17. The 

 second turn of the cochlea; the figure is placed upon the lamina spiralis, and therefore 

 in the scala vestibuli, the scala tympani being beneath the lamina. 18. The remaining 

 half turn of the cochlea; the figure is placed in the scala tympani. 19. The lamina 

 spiralis terminating in its falciform extremity. The dark space included within the 

 falciform curve of the extremity of the lamina spiralis is the helicotrema. 20. The in 

 fundibulum. 



* Cotunnius regarded these processes as tubular canals, through which the superabuft 

 dant aqua labyrinthi might be expelled into the cavity of the cranium. Mr. WhartoA 

 Jones, in the article " Organ of Hearing," in the Cyclopaedia of Anatomy and Physiology, 

 also describes them as tubular canals which terminate beneath the dura mater of the 

 petrous boue in a small dilated pouch. In the ear of a man, deaf and dumb from hirtli, 

 he found the termination of the aqueduct of the vestibule of unusually large size mcoB 

 sequence of irregular development. 



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