NORMAL HISTOLOGY. 



The skin covering the greater part of the bony canal, on the 

 contrary, is very thin and intimately united to the periosteum. 



Hairs and glands are want- 



FIG. 393. ing in this part of the canal, as 



they are also in the integument 

 reflected over the external sur- 

 face of the tympanic membrane. 

 The membrana tympani 

 consists of three layers : (i) the 

 central ground-stratum, or lam- 

 ina propria, composed of 

 fibrous connective tissue, (2) 

 the cutaneous layer reflected 

 over the external surface of the 

 drum, and (3) the mucous 

 layer covering the inner side 

 of the membrane as the repre- 

 sentative of the lining of the 

 tympanic cavity. 



The tegumental layer con- 

 sists of the usual epidermis 

 and connective-tissue corium, 

 the latter being only about half 

 as thick as the epithelial layer. 



The central connective-tissue 

 ground-plate, or lamina pro- 

 pria, constitutes the fibrous basis 

 of the tympanic membrane and 



represents its mesodermic portion. This layer consists of closely- 

 felted bundles of fibrous tissue arranged as two strata, the outer or 

 radial fibre-layer, composed of fibrous bundles, which in their 

 general course radiate from the periphery of the tympanum towards 

 the point of attachment of the head of the malleus, and the inner or 

 circular fibre-layer, consisting of concentrically-disposed bundles, 

 whose greatest development is at the periphery in the vicinity of the 

 annular attachment of the membrana tympani. 



The mucous layer is a part of the general lining of the middle 

 ear, and consists of a thin connective-tissue tunica propria or 

 groundwork, composed of delicate bundles of fibre-elastic tissue, 

 upon which rests the epithelium ; the latter consists of a single 

 layer of low cuboidal polyhedral cells without cilia. 



The blood-vessels supplying the tympanic membrane are derived 

 from two sources, the one set proceeding from the branches of the 

 external auditory canal to end in capillaries which ramify within the 



Section of bony portion of human external audi- 

 tory canal : s, cutaneous layer closely united with 

 periostea! fibrous tissue ; o, osseous tissue of wall. 

 (After Riidinger.) 



