1494 HUMAN ANATOMY. 



The Membrana Tympani. — The tympanic membrane or drum-head is a 

 delicate transparent disc, irregularly oval or ellipsoidal in outline and concave on its 

 outer surface. It is placed obliquely with the horizontal plane, forming an angle of 

 about 55°, opening outward. As the middle portion of the membrane is drawn in- 

 ward, the inclination of its several parts differs. The obliquity of the membrane 

 is about the same in the infant as in the adult. With the upper back wall of the 

 external auditory canal the drum-head forms a very obtuse angle, whilst with the 

 antero-inferior wall it encloses an angle of about 27°. The longest diameter of the 

 membrane is directed from above and behind, forward and downward, and measures 

 from 9.5-10 mm. ; the shortest is from 8.5-9 m'^''- The membrane is about . 10 mm. 

 thick, except at the periphery, where it is thickened. Like the rest of the tympa- 

 num and the labyrinth, it is practically as large in the infant as in the adult. 



The handle of the malleus is embedded in the tympanic membrane (Fig. 

 1252), and extends from a point near the middle, upward and forward toward the 

 periphery, to end at the short process. At its lower end, the handle of the 

 malleus is flattened laterally and broadened at the umbo, which corresponds to the 

 deepest part of the concavity of the membrane. The short process of the malleus 

 forms a conspicuous rounded projection at the antero-superior part of the drum- 

 head. Extending from the short process of the malleus to the anterior and 

 posterior ends of the tympanic ring are two straight striae. The part of the 

 drum-head included between these striae and the Rivinian notch is known as the 

 membrana flaccida (pars flaccida) or Shrapnell's membrane. It is thinner 

 and less tense than the remaining larger part of the drum-head which is called the 

 membrana tensa (pars tensa). 



The inner aspect of the drum-head presents two folds of mucous membrane which 

 stretch horizontally backward and forward to the annulus and form an anterior and a 

 posterior \\\v&cX.^A pocket. The anterior pocket contains in its wall, in addition to the 

 mucous membrane, the long process of the malleus, the chorda tympani nerve and 

 the inferior tympanic artery, the nerve continuing along the lower border of the 

 posterior fold. 



The structure of the tympanic membrane includes three main layers: (i) the middle 

 fibrous stratum, or uievibrana propria / ( 2 ) the external cutafieous layer, the prolongation of 

 the skin lining the e.xternal auditory canal ; and (3) the internal mucous metnbrane, a continua- 

 tion of the mucous membrane clothing other parts of the tympanic cavity. 



The fibrous layer or membrana propria represents the mesoblastic portion of the drum-head 

 and consists of an outer stratum of radially disposed fibres which diverge from the malleus 

 towards the periphery of the membrane, and an inner stratum of circular fibres, concentrically 

 arranged and best developed near the periphery of the membrane but absent at the umbo. The 

 radiating fibres, on the contrary, become more dense at the umbo, partly through accumulation 

 and partly through splitting (Gerlach). Between the fibres of the two layers are seen connect- 

 ive tissue corpuscles which are spindle-shaped in longitudinal and stellate in cross-section. 



The membrana propria is absent within the pars flaccida or Shrapnell's membrane. At the 

 periphery of the membrana propria, the fibres, especially those of the radial stratum, are con- 

 nected with those of a ring of thick connective tissue, the annulus fibrosus which occupies the 

 sulcus tympanicus. The fibres of the annulus fibrosus run in various directions, but for the 

 most part converge toward the tympanic membrane proper (Fig. 1253). Round cells are 

 found between these fibres. 



The cutaneous layer consists of a thin epidermal stratum, composed of two or three rows 

 cf cells and a delicate sheet of connective tissue, but neither a definite corium nor papillce are 

 present. A thickened band of subepithelial connective tissue extends across Shrapnell's mem- 

 brane and along the handle of the malleus and contains the large vessels and nerves which pass 

 from the meatus to the membrana tympani. 



The mucous membrane covering the inner surface of the drum-liead consists of a scanty 

 layer of connective tissue, invested with a sheet of large low nonciliated epithelial cells. 



The vessels of the tympanic membrane include arteries which are arranged as an outer and 

 inner set, separated by the membrana propria. The former set is derived from the deep auricu- 

 lar branch of the internal maxillary artery ; tfie latter from the tympanic branch of the internal 

 maxillary and from the stylo-mastoid branch of the posterior auricular. Each of these sets 

 forms a plexus of vessels with a large branch extending downward along the malleus handle, 

 and another around the periphery of tiie membrane, these two branches being connected by 

 numerous radiating twigs. Perforating vessels connect the two sets of arteries, especially along 



