THE ORGAN OF HEARING. 381 



by a single layer of low polyhedral cells, is very thin and inti- 

 mately united with the delicate periosteal layer ; numerous fibres, 

 trabeculae, or lamellae pass between neighboring surfaces and partially 

 occlude the spaces within the bone, thereby reducing the lumina and 

 still further adding to the complexity of the mastoid cells. 



The secondary tympanic membrane, closing the fenestra ro- 

 tunda, consists of three layers, a central fibrous lamina propria, 

 which is covered on the tympanic surface by a reflection of the mu- 

 cous membrane, and on the other side by the extension of the 

 lining of the vestibular perilymphatic space. The lamina propria, 

 the unossified part of the wall of the labyrinth, is composed of ra- 

 dially-disposed bundles of fibrous tissue passing from the indented 

 point of its base towards the periphery. The mucous stratum is 

 formed of a thin fibrous tunica propria invested by a single layer 

 of flattened non-ciliated polyhedral epithelial cells, similar to 

 those covering the neighboring promontorium. The innermost 

 stratum of the membrane is composed of a thin layer of sub- 

 endothelial fibrous tissue, over which extends the single layer 

 of endothelial plates. 



The larger blood-vessels supplying the mucous lining of the 

 tympanum lie within the deeper periosteal layer of the mucosa and 

 give off smaller branches, which pass superficially to form a capillary 

 net-work beneath the epithelium. The vessels distributed to the 

 mucosa covering the promontorium are remarkable for the absence 

 of anastomoses, the arteries dividing into twigs possessing rela- 

 tively large lumina ; the terminal arterioles pass very rapidly into 

 venous radicles, so that intervening capillaries scarcely exist, in places 

 being entirely wanting. 



The lymphatics of the tympanic mucous membrane form a sys- 

 tem of channels within the deep periosteal layer, where the lymph- 

 vessels are supplemented by spherical enlargements and lateral dila- 

 tations. The reticular connective tissue of the mucosa exhibits local 

 accumulations of lymphoid cells, which strongly suggest the 

 presence of lymphatic nodules. 



The principal nerves of the tympanum, derived from the tym- 

 panic plexus, run within the periosteal layer of the mucosa, and are 

 composed almost entirely of medullated fibres. From the deeper 

 trunks fine twigs pass towards the surface and form a wide-meshed 

 plexus, which contributes delicate bundles of pale non-medullated 

 fibres to a subepithelial net-work. Along the course of the 

 larger trunks and their immediate branches groups of ganglion-cells 

 occur in exceptional cases, these being found in proximity with the 

 epithelium. 



The ear-ossicles consist of compact bone, in which Haversian 



