SECT. 233.] THE INTERNAL EAR. 587 



twigs (v. Troltsch). Pale nerve-fibres have been seen by Gerlach 

 in the mucous covering of the tympanic membrane. 



§ 233. The vestibule and the osseous semi-circular canals are 

 covered on their inner surface by an extremely thin periosteum, 

 consisting of rigid connective tissue, with fine fibres and numerous 

 nuclei, but without any elastic elements ; it resembles in many 

 respects the fibrous structures of the inner wall of the canal of 

 Schlemm in the eye. Upon the surface of this periosteum, is 

 seated a simple pavement epithelium of delicate polygonal nucle- 

 ated cells, measuring 0-007'" to O'ooo/", which are concerned with 

 the scanty vessels of the periosteum in the formation of the peri- 

 lympha s. aqua Cotunni, which fills the osseous labyrinth. — By 

 the junction of the periosteum of the labyrinth with the lining of 

 the tympanic cavity, is produced the structure known as the 

 membrana tympani secundaria, which, like the true tympanic 

 membrane, is composed of a middle fibrous layer containing vessels 

 and a few nervous filaments, together with two epithelial layers. 



The two membranous saccules and the membranous canals, con- 

 tained in the interior of the vestibule, and of the osseous semi- 

 circular canals, all present essentially the same structure. Their 

 walls are firm transparent and elastic, and of some thickness 

 (0-012"' to 0-015'" in the canals, o'Oi6"' in the saccules) in 

 proportion to the diminutiveness of the parts; they present ex- 

 ternally a membrane composed of fine reticulate fibres, resembling 

 very much the lamina fusca or outer pigment layer of the choroid, 

 and even possessing, like it, some irregularly scattered brownish 

 pigment cells. Then follows a transparent vitreous membrane, 

 0-004'" to o - oo8'" in thickness, and sharply bounded internally; 

 here and there this membrane exhibits a distinct delicate longitu- 

 dinal striation, and a number of elongated nuclei are always to be 

 seen on the addition of acetic acid ; hence it is probably not to be 

 classed with the membrana proprice, such as the capsule of the 

 lens for instance, though it approaches these membranes very 

 much, even in its chemical reactions. Lastly, the innermost layer 

 is a simple pavement epithelium, 0-003'" thick, breaking down 

 readily into its elements, with polygonal cells of various sizes 

 (0-004'" to 0008'"); it lines all the spaces mentioned, and encloses 

 a fluid named endo-lympha s. aquula vitrea auditiva : in this fluid 

 of the ear of fishes, Barruel has established the occurrence of 

 mucus. 



The vessels of the membranous labyrinth are pretty numerous, 



