518 DISTRIBUTION OF THE AUDITORY NERVE. 



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 membrane of the labyrinth by the aqua labyrinth!. In 

 structure it is composed of four layers : an external or serous layer, 

 derived from the lining membrane of the labyrinth ; a vascular layer, 

 in which an abundance of minute vessels are distributed ; a nervous 

 layer, formed by the expansion of the filaments of the vestibular nerve 

 and an internal and serous membrane, by which the limpid fluid which 

 fills its interior is secreted. Some patches of pigment have been ob- 

 served by Mr. Wharton Jones in the tissue of the membranous laby- 

 rinth of man. Among animals such spots are constant. 



The membranous labyrinth is filled in its interior with a limpid 

 fluid, first well described by Scarpa, and thence named liquor Scarpae* 

 (endolymph, vitreous humour of the ear), and contains two small cal- 

 careous masses called otoconites. The otoconites (ovj, urog xw?, the 

 ear-dust), consist of an assemblage of minute, crystalline particles of car- 

 bonate and phosphate of lime, held together by animal substance, and 

 probably retained in form by a reflection of the lining membrane of 

 the mebranous labyrinth. They are found suspended in the liquor 

 Scarpae ; one in the sacculus communis, the other in the sacculus 

 proprius, from that part of each sac with which the nerves are con- 

 nected. 



The AUDITORY NERVE divides into two branches at the bottom of 

 the cul de sac of the meatus auditorius internus; a vestibular nerve, 

 and a cochlear nerve. The vestibular nerve, the most posterior of 

 the two, divides into three branches, superior, middle, and inferior. 

 The superior vestibular branch gives off a number of filaments which 

 pass through the minute openings of the eminentia pyramidalis and 



the vestibule, and that below it in the inferior ventricle. 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 extre- 

 mity of the scala tympani corresponding with the fenestra rotunda. 15. The 

 lamina spiralis ; the figure is situated in the scala vestibuli. 16. The open- 

 ing 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 re- 

 maining half turn of the cochlea; the figure is placed in the scala tympani. 

 19 The lamina spiralis terminating in a falciform extremity. The dark space 

 included within the falciform curve of the extremity of the lamina spiralis is 

 the helicotrema. 20. The infundibulum. 



* Antonio Scarpa is celebrated for several beautiful surgical and anatomi- 

 cal monographs; as, for example, his work on ' Aneurism." " De Auditu et 

 Olfactu," &c. An account of the aqua labyrinthi will be found in his anato- 

 mical observations " De Structura Fenestne Rotundse, et de Tympano 

 Secundario." 



