THE EAR 1541^ 



ating fibres of the membrane proceed. This knob is firmly attached, 

 and, being directed inwards, the membrane is consequently drawn 

 inwards at that point, and its outer surface presents a slight conical 

 depression, the deepest part of which is called the umbo. 



Structure. — ^The membrana tympani consists of three layers — 

 external, middle, and internal. 



The external or cutaneous layer is very thin, and is derived from 

 the skin of the meatus auditorius extemus. It contains no glands, 

 is freely provided with bloodvessels and nerves, and is covered by 

 stratified squamous epithelium. 



The middle or fibrous layer forms the proper substance of the 

 membrane, and consists of fibrous tissue. The fibres are arranged 

 in two sets — radial and circular. The radial fibres lie beneath the 

 cutaneous layer, and radiate from the handle of the malleus to the 

 annulus fibrosus. The circular fibres are situated within the radial 

 fibres, and are most numerous towards the circumference of the 

 membrane. Both sets of fibres are absent from the membrana 

 flaccida. 



The Internal or mucous layer is continuous with the mucous mem- 

 brane of the tympanum, and is covered by a single layer of squamous 

 epithelium. 



The membrana flaccida, or Shrapnell's membrane, is composed of 

 two layers — cutaneous and mucous, there being no fibrous layer. 

 These two layers are united by connective tissue, which is Very 

 loosely arranged, and this renders the membrane flaccid. This 

 part is very liable to perforation. 



Cone of Light. — Extending from the knob, in which the handle of 

 the malleus terminates, downwards and inwards to the antero- 

 inferior margin of the membrana tympani there is seen a specially 

 bright reflection, triangular in outline, with the apex towards the 

 umbo. This is called the cone of light. 



Arterial Supply of the Membrana Tympani. — (i) Deep auricular 

 branch of the internal maxillary artery. This vessel, which passes 

 through the anterior wall of the meatus auditorius extemus, supplies 

 the cutaneous layer. It descends from the skin of the roof of the 

 meatus along the course of the handle of the malleus to the umbo, 

 where it divides into branches which radiate towards the circum- 

 'ference of the membrane. (2) The stylo-mastoid branch of the 

 posterior auricular, and (3) the tympanic branch of the internal 

 maxillary. The former artery enters the tympanum from the 

 aqueduct of Fallopius, and the latter through the fissure of Glaser. 

 Branches from them supply the mucous layer, and form an anasto- 

 motic ring around the circumference of the membrane. The 

 fibrous layer receives its arterial supply from the vessels of the 

 cutaneous and mucous layers. 



The veins pass to the external jugular and internal jugular 

 veins. 



Nerve-supply. — The nerves are derived from (i) the auriculo- 

 temporal of the inferior maxillarj^ (2) the auricular branch (Arnold's 



