THE EAR. 769 



are scantily distributed in the mucous membrane and membrana tympani. 

 With their terminations we are unacquainted, whilst it is known that 

 the tympanic nerve contains numerous large ganglion-cells, either 

 isolated or aggregated into small ganglia. 



234. The inner surface of the vestibule and of the osseous semicir- 

 cular canals is lined by an extremely thin periosteum, consisting of a 

 rigid, finely-fibrous connective tissue, without elastic fibres, but with nu- 

 merous nuclei and in many respects resembling the forms of fibre met 

 with in the inner wall of the "canal of Schlemm" in the eye. On the 

 surface of this periosteum rests a tessellated epithelium in a single layer, 

 with delicate, polygonal, nucleated cells of 0-007-0-009 of a line, which, 

 as well as its certainly not very numerous vessels, stands in relation to 

 the perilympha s. aqua Cotunni, filling the osseous labyrinth. By the 

 conjunction of the periosteum of the labyrinth and the lining of the 

 tympanum, are produced the membranes tympani secundarice, which, 

 like the proper membr. tympani, are composed of a middle fibrous layer 

 with vessels and scattered nervous filaments, and two epithelial layers. 



The two sacculi and canals, contained in the interior of the vestibule 

 and of the osseous semicircular canals, all essentially present the same 

 structure. Their firm, transparent, and elastic 

 walls, which are tolerably thick in proportion to 

 the minuteness of the parts (0-012-0-015 of a 

 line in the canals, and 0-016 in the sacculi), 

 present, most externally, a membrane com- 

 posed of reticulated, fine fibres, approaching 

 very nearly to the outer colored layer of the 



choroid or the lamina fusca, like which it also occasionally contains irre- 

 gular brownish pigment-cells. This is succeeded, by a transparent, 

 glassy membrane, sharply defined, especially on the inner aspect, 0-004- 

 0-008 of a line thick, presenting in part a distinct, delicate, longitudinal 

 striation, and on the addition of acetic acid always exhibiting a multi- 

 tude of elongated nuclei, and which consequently cannot well be placed 

 in the same class with the membrana? proprice, the capsule of the lens, 

 &c., although it very nearly approaches them in its chemical reactions. 

 The innermost layer, lastly, is a simple, readily disintegrated, tessellated 

 epithelium, 0-003 of a line, with sometimes larger, sometimes smaller 

 (0-004-0-008 of a line) polygonal cells, lining all the spaces in question, 

 and enclosing the so-termed endolymph. s. aquula vitrea auditiva, in 

 which, in Fishes, Barruel has ascertained the presence of mucus. 



The vessels of the membranous labyrinth are tolerably numerous, and 

 are distributed in minute arteries and veins and abundant capillary net- 



FiG. 308. Transverse section of a semicircular canal, magnified 250 diameters : a, fibrous 

 membrane with nuclei ; - 6, homogeneous membrane ; c, epithelium. From the Calf. 



49 



