810 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



drawn from the centre to the margin of the membrane would not be straight, 

 but would be curved slightly, with the convexity outward, this shape being 

 due to the tension of the elastic circular fibres of the membrane. The mem- 

 brane, throughout the greater part of its circumference, is inserted in a groove 

 in a bony ring set in the wall of the auditory canal, but a small arc at its 

 superior portion is attached directly to the wall of the canal. The segment of 

 membrane corresponding to this arc, known as the membrana fiaccida, lacks 

 the tenseness of the rest of the drum-skin. 



Viewed through the aural speculum, the normal tympanic membrane has 

 a pearly lustre (Fig. 268). The handle of the malleus, or manubrium, inserted 

 within its fibrous layer, can be seen as an opaque ridge running from near the 

 upper anterior margin downward and backward and ending in the umbo, or 

 central depression, where the membrane is drawn considerably inward by the 

 tip of the manubrium. It is from this point that the radial fibres of the mem- 

 brana propria diverge. 



At the top of the manubrium is a shining spot which is the reflection 

 from the short process of the malleus where it presses against the membrane. 

 From this point two delicate folds of the membrane run to the periphery 

 one forward and the other backward. They form the lower border of the 

 membrana fiaccida, or Shrapnell's membrane, in which there is less fibrous tissue 

 than in the remaining part of the membrane, and the cutaneous and mucous 

 layers are also less tense than elsewhere. A bright reflection of triangular 

 shape, known as the " pyramid of light," is seen in the lower quadrant of the 



tympanic membrane. The apex of this- 

 bright triangle is at the tip of the manu- 

 brium, and its base is on or near the 

 periphery of the membrane. 



Auditory Ossicles. The tympanic 

 membrane is put into relation with the 

 internal ear by a chain of bone, the 

 auditory ossicles, known as the malleus,. 

 the incus, and the stapes, so called from 

 their fancied resemblance to a hammer, an 

 anvil, and a stirrup (Figs. 267, 269, 270). 

 The malleus (Fig. 271) is 18 to 19 milli- 



meterS lon g '> {i P^Sents a rounded head, 



grooved OH One side for articulation With 



. * - -, , -, -, * T * 



the mclls > a short neck > and a lon g handle 



O r manubrium. which is inserted in the 



. _ 



tissue of the tympanic membrane from 

 a point on its upper periphery to a little below its centre. The processus 

 brevis of the malleus is a low conical projection which springs from the top 

 of the manubrium and presses directly against that segment of the tympanic 

 membrane known as the membrana flaccida, through which it can be seen 

 shining on inspection with the ear-speculum. The processus gracilis, or pro- 



PIG. 271.-Malleus of the right side : A, anterior 

 face ; B, internal face (after Testut) : 1, capitu- 

 lum or head of malleus ; 2, cervix or neck ; 3, 

 processus brevis ; 4, processus gracilis ; 5, manu- 



brium; 6, grooved articular surface for incus; 



7, tendon of m. tensor tympani. 



