THE EAR. 405 



the largest arteries and veins run along the manubrium of the 

 malleus in the middle coat, constituting arterial and venous cir- 

 cles around the periphery of the membrane, and also ramifying 

 abundantly in the mucous membrane. The nerves are derived 

 principally from the ninth and fifth pairs, and upon the whole 

 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 semi- 

 circular canals is lined by an extremely thin periosteum, con- 

 sisting of a rigid, finely-fibrous connective tissue, without 

 elastic fibres, but with numerous nuclei and in many respects 

 resembling the forms of fibre mejt with in the inner wall of the 

 "canal of Schlemm" in the eye. On the surface of this 

 periosteum rests a tesselated epithelium in a single layer, with 

 delicate, polygonal, nucleated cells of 0*007 — 0-009"', 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 laby- 

 rinth and the lining of the tympanum, are produced the 

 membrance tympani secundaria, 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 ^-^p^is^ » 



tolerably thick in proportion to the minute- ^2J5" ™^^fe^ 

 ness of the parts (0-012 — 0'15"' in the 

 canals, waft. 0-01 6'" in the sacculi), present, 

 most externally, a membrane composed of 

 reticulated, fine fibres, approaching very 

 nearly to the outer coloured layer of the choroid or the lamina 

 fusca, like which it also occasionally contains irregular brownish 



Fig. 308. Transverse section of a semicircular canal, x 250 diara. : a, fibrous 

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



