TYMPANIC MEMBRANE 



263 



ring at the margin of the membrane. The manubrium or handle of the 

 malleus (Fig. VII-4) is inserted between the mucous membrane and the 

 inner fibrous layer. 



The radial fibers terminate at the external lower end of the manu- 

 brium. When the tensor tympani muscle contracts, the handle of the 



Top in standing position 



Malleus 



Tensor tympani 



Anterior 



ncus 



Posterior 



of 

 tympani 



Fig. VII-4. A semi-diagrammatic view of the malleus-incus articulation looking 

 at the inner convex surface of the eardrum. The lower figure is a diagonal section 

 through the tympani showing the attached malleus. 



malleus and the tympanic membrane are pulled inward, forming a 

 shallow, unsymmetrically indented dish with a rolled edge and an eccen- 

 tric hollow in its outer surface termed the umbo. Owing to the unsym- 

 metrical suspension of the eardrum and its attachment to the manu- 

 brium, a fold of the membrane just below the head of the malleus is 

 left in a flaccid state. 



Helmholtz [1862] described the membrane as pulled tight but slightly 

 curved outward at its edge and so constructed that a decrease in external 

 pressure should increase this curvature. A large inward change in curva- 

 ture at the center should produce a pronounced change just inside the 

 fibrocartilaginous edge, where a circular layer of fibrous tissue strengthens 

 the membrane. The circular fibrous layer is thickest close to the manu- 

 brium attachment. Between these two regions this layer is partially 

 missing. Just within the fibrocartilage ring this layer ends abruptly. 



