THE EXTERNAL EAR. 31 



of the malleus, and the ineridianal lines of which are arched in a 

 convex manner towards the cavity. 



The examination of the intact membrane with low powers 

 of the microscope will be found very serviceable in under- 

 standing the topography of its several constituent elements. 

 With this object in view, the membrane should be pre- 

 pared with the bony margins and the ossicula in situ, de- 

 tached from the temporal bone, and placed for a few hours 

 in water, when the cuticle which obstructs the view can be in 

 great measure removed. The preparation is then to be de- 

 prived of water by immersion in absolute alcohol, rendered 

 transparent by oil of turpentine, and allowed to dry. With 

 the aid of low powers, three layers may now be distinguished, 

 an external, a middle, and an internal, which are adherent by 

 means of a thickened border, the tendinous ring, t6 a bony 

 groove, which fails only at the Rivinian hiatus. The external 

 layer, which is to be regarded as a continuation of the cutis of 

 the auditory meatus, agrees essentially with this in its struc- 

 ture. The middle layer, which is the thickest of the three, 

 consists of sharply defined fibres of various breadth, the 

 greater number of which run either in a radial or circular 

 direction to the malleus ; only a small portion diverges between 

 the two former in the most various directions. The radial 

 layer lies externally beneath the cutis ; the circular internally 

 beneath the mucous membrane. 



The internal, or muco-membranous layer of the membrana 

 tympani, is an immediate prolongation of the mucous mem- 

 brane of the tympanic cavity. It is very thin, and, on ac- 

 count of its complex structure, can only be distinguished with 

 high powers. Although it is easy to demonstrate the different 

 arrangement of the elements composing the tympanic mem- 

 brane, an exception occurs at one spot, the Rivinian hiatus, 

 respecting which there is still much difference of opinion. The 

 bony groove into which the membrana tympani is inserted 

 does not return into itself. A notch occurs in the bone, 

 in the form of a more or less flattened abscission of the circle, 

 the chord of which, having a length of 2*5 to 3 millimeters, is 

 represented by the connecting line of the two ends of the 



