chap. XL.] THE OUTER AND MIDDLE EAR. 319 



also arranged in a circular direction. The former 

 belong to the outer, the latter to the inner, portion of 

 the middle stratum. 



The mucous membrane lining the tympanic surface 

 of the membrane is delicate connective tissue, covered 

 with a single layer of polyhedral epithelial cells. 



The blood-vessels form capillary networks for all 

 three layers i.t., a special network for the skin 

 layer, a second for the middle stratum, and a third one 

 for the mucous layer ; the lymphatics are also arranged 

 in this way. An intercommunicating system of lym- 

 phatic sinuses and clefts (Kessel) is left between the 

 trabeculse. The non-medullated nerve-fibres form 

 plexuses for the skin and mucous layer ; from these 

 pass off fine fibrils, which form a sub-epithelial net- 

 work, and from this the fibrils pass into the epithelium. 



455. The tuba distachii is lined with a 

 mucous membrane, which is a continuation of that 

 lining the upper part of the pharynx, and therefore, 

 like it, is covered on its inner or free surface with 

 columnar ciliated epithelium. As in the pharynx, so 

 also here, we find a good deal of adenoid tissue in the 

 mucous membrane. 



The cartilage of the tuba Eustachii in the adult ap- 

 proaches in structure the elastic cartilages of other parts. 



456. The cavuni tympani, including the cellulae 

 mastoidese and the surface of the ossicula auditus, is 

 lined with a delicate connective tissue membrane. Its 

 free surface is covered with a single layer of poly- 

 hedral epithelial cells in the following regions : on 

 the promontory of the inner wall of the cavity, on the 

 ossicula auditus, on the roof of the cavity, and in the 

 cellulse mastoidese ; in all other parts it is columnar cili- 

 ated epithelium, like that lining the tuba Eustachii. 



457. The three ossicula auditus are osseous 

 substance covered with periosteum, which is covered 

 with the delicate mucosa just described. The liga- 



