458 



TYMPANUM. 



Fig. 204.* 



MIDDLE EAR OR TYMPANUM. 



The tympanum is an irregular bony cavity, compressed from without 

 inwards, and situated within the petrous bone. It is bounded, externally , 

 by the meatus and membrana tympani ; internally, by the base of the pe- 

 trous bone ; behind, by the mastoid cells ; and, throughout the rest of its 

 circumference, by the thin osseous layer which connects the petrous with 

 the squamous portion of the temporal bone. 



The Membrana tympani is a thin 

 and semi-transparent membrane of 

 an oval shape, its long diameter be- 

 ing vertical. It is inserted into a 

 groove situated around the circum- 

 ference of the meatus, near its termi- 

 nation, and is placed obliquely across 

 the area of that tube, the direction 

 of the obliquity being downwards 

 and inwards. It is concave towards 

 the meatus, and convex towards the 

 tympanum, and is composed of three 

 layers, an external, epidermal ; mid- 

 dle, fibrous and muscular ; and, in- 

 ternal, mucous, derived from the 

 mucous lining of the tympanum. 



The tympanum contains three 

 small bones, ossicula auditus, viz., 

 the malleus, incus, and stapes. 



The Malleus (hammer) consists of 

 a head, neck, handle (manubrium), 



and two processes, long (processus gracilis), and short (processus brevis). 

 The manubrium is connected with the membrana tympani by its whole 

 length, extending below the central point of that membrane. It lies be- 

 neath the mucous layer of the membrane, and serves as a point of attach- 

 ment to which the radiating fibres of the fibrous layer converge. The long 

 process descends to a groove near the fissura Glaseri, and gives attachment 

 to the laxator tympani muscle. Into the short process is inserted the ten- 

 don of the tensor tympani, and the head of the bone articulates with the 

 incus. 



The Incus (anvil) is named from an imagined resemblance to an anvil. 

 It h^s also been likened to a bicuspid tooth, having one root longer than, 



*A diagram of the ear. p. The pinna, t. The tympanum. 1. The labyrinth. 1. 

 The upper part of the helix. 2. The antihelix. 3. The tragus. 4. The autitragus. 5. 

 The lobulus. 6. The concha. 7. The upper part of the fossa innominata. 8. The 

 meatus. 9. The membrana tympani, divided by the section. 10. The three little bones, 

 crossing the area of the tympanum, malleus, incus, and stapes; the foot of the stapes 

 blocks up the fenestra ovalis upon the inner wall of the tympanum. 11. The promon- 

 tory. 12. The fenestra rotunda; the dark opening above the ossicula leads into the 

 mastoid cells. 13. The Eustachian tube; the little canal upon this tube contains the 

 tensor tympani muscle in its passage to the tympanum. 14. The vestibule. 15. The 

 three semi-circular canals, horizontal, perpendicular, and oblique. 16. The ampullae 

 upon the perpendicular and horizontal canals. 17. The cochlea. 18. A depression be- 

 tween the convexities of the two tubuli which communicate with the tympanum *nd 

 vestibule : the one is the scala tympeni, terminating at 12 ; the other is the sc*la 

 vostibuli, 



