THE MIDDLE EAR. 1501 



inconstant folds of mucous membrane extend from the wall of the tympanum to 

 the malleus and the incus. The most constant of these is the outer malleo-incudal 

 plica, which stretches backward to the posterior ligament of the incus. Additional 

 folds frequently extend between the crura of the stapes and from these to the wall 

 of the tympanum. 



The epithelium of the tympanic mucosa varies in different parts of the cavity. 

 Over the promontory, the ossicles and the tympanic membrane, it consists of a single 

 layer of low cuboidal nonciliated cells, whilst over the other parts the cells are ciliated 

 columnar in type. Small tubular glands occur within the lining of the anterior part 

 of the cavity. The subepithelial connective tissue, which supports the vessels and 

 nerves, comprises two layers, the outer forming the periosteum of the bony wall. 



The secondary tympanic membrane closing the fenestra cochleae, bulges 

 somewhat toward the cochlea and is attached to the bony crest or ridge of the win- 

 dow by its widened rim. It consists of three layers, of which the middle one is a 

 distinct fibrous lamina propria, which is covered on the tympanic surface by mucous 

 membrane, and on the other side by an extension of the lining of the perilymphatic 

 space. The lamina propria is composed of radially disposed bundles of fibrous 

 tissue. The outer mucous stratum is formed of a thin fibrous tunica propria, 

 invested by a single layer of flattened nonciliated epithelial cells, similar to those 

 covering the neighboring promontory. The innermost stratum of the membrane 

 includes a thin layer of subendothelial fibrous tissue, over which stretches a layer of 

 endothelial plates. 



Vessels and Nerves of the Tympanum. — The arteries supplying the 

 tympanic cavity are from five sources. 



1. The stylo-mastoid branch'of the posterior auricular artery passes through the 

 stylo-mastoid foramen and the Fallopian aqueduct, and sends a branch to the sta- 

 pedius muscle and three branches to the posterior part of the tympanic cavity. One 

 of these passes to the floor, one through the canal for the chorda tympani nerve, and 

 one to the posterior part of the oval window. 



2. The tympanic branch of the internal maxillary artery enters the tympanic 

 cavity through the Glaserian fissure and supplies the anterior part of the cavity, 

 including the anterior ligament of the malleus, the processus gracilis and the tympanic 

 membrane. 



3. The middle meningeal branch of the internal maxillary artery sends a branch 

 through the hiatus Fallopii to anastomose with the stylo-mastoid artery, a branch 

 through the canaliculus tympanicus to the promontory, and a branch to the tensor 

 tympani muscle. 



4. The ascending pharyngeal sends branches to the floor and the promontory, 

 one of them accompanying Jacobson's nerve. 



5. The internal carotid artery in its passage through the carotid canal gives off 

 branches to the anterior wall of the tympanic cavity. 



The veins follow, in a general way, the course of the arteries. They are tribu- 

 tary to the middle men.ingeal, the pharyngeal plexus and the jugulars. 



The lymphatics arise from a net- work within the mucous membrane and end 

 chiefly in the retropharyngeal and the parotid nodes. 



The nerves supplying the mucous membrane of the tympanum are branches 

 from the tympanic plexus formed by the tympanic branch of the glosso-pharyngeal 

 nerve, in conjunction with sympathetic filaments from the net-work accompanying the 

 carotid artery. The tensor tympani muscle receives its supply from the trigeminus; 

 the stapedius muscle from the facial. Although the chorda tympani nerve has an 

 intimate topographical relation to the space, which it traverses close to the outer 

 wall, it gives no filaments to the structures within the tympanum. 



The Eustachian Tube. 



The Eustachian tube (tuba auditivaj is a canal, partly bony and partly cartilagi- 

 nous, extending from the lateral wall of the naso-pharynx backward, upward and out- 

 ward to the anterior part of the tympanum. In the adult it measures about 37 mm. 

 CiJ^ in. J in length, of which approximately the upper third (tympanic portion) 



