380 NORMAL HISTOLOGY. 



lower third of the process into two terminal twigs. In addition to 

 these central nerves, small stems enter the drum-membrane at 

 various points at the periphery, both sets of twigs taking part in 

 the formation of a wide-meshed ground-plexus. From the latter 

 fine pale fibres pass to the blood-vessels which they surround, while 

 other fibres extend to the superficial part of the layer, where, be- 

 neath the epidermis, they constitute a subepithelial plexus. The 

 nerves of the mucous layer originating in the tympanic plexus are 

 largely distributed to the lymphatics as well as to blood-vessels ; 

 an additional subepithelial plexus bears close relations to the epi- 

 thelium ; a few fibres extend into the fibrous tissue of the lamina 

 propria. 



THE MIDDLE EAR. 



The middle ear, the entodermic division of the auditory appa- 

 ratus, comprises the tympanic cavity, with its extension into the 

 mastoid cells, and the Eustachian tube, together with the series 

 of minute ear-ossicles. 



The walls of the tympanic cavity consist of the surrounding 

 bony structures with their periosteum, over which is reflected the 

 mucous lining, indirectly continuous with that of the pharynx. The 

 mucous membrane, closely united with the underlying periosteum, 

 not only covers the inner surface of the membrana tympani, but is 

 also reflected over the ear-bones and their ligaments as well as over 

 the nerves and blood-vessels crossing the cavity. The mucosa con- 

 sists of a thin fibrous tunica propria (50-60 //) which in places 

 resembles the reticulum of adenoid tissue and includes leucocytes ; 

 the mucous layer is intimately blended with the denser fibrous struct- 

 ure of the periosteum. Connected with the trabeculse of the mu- 

 cosa peculiar oval bodies are occasionally encountered, which are 

 composed of an axial band and concentric lamellae of connective 

 tissue ; these bodies are normal but probably not constant constit- 

 uents of the middle ear. 



The epithelial lining (18-21 //.) of the tympanic cavity differs in 

 character in the several regions ; over the ear-ossicles, the tympanic 

 membrane, and the promontory, as well as within the mastoid cells, 

 the epithelium consists of a single layer of low cuboidal po- 

 lygonal cells without cilia ; over the remaining parts of the mid- 

 dle ear a layer of ciliated columnar cells exists. In those places 

 where nerve-trunks or blood-vessels are covered, the greatly-thick- 

 ened mucosa forms local ridges, within which the trunks are en- 

 closed. Small tubular glands, about . i mm. in length, occur in the 

 mucous membrane of the anterior part of the tympanic cavity ; they 

 are sparingly distributed and subject to individual variation. 



The mucous lining of the antrum and the mastoid cells, clothed 



