CAVITY AND MEMBRANE OF THE TYMPANUM. 745 



sideration a roof and a floor, an outer and an inner wall, and an anterior 

 and a posterior boundary. 



The roof of the tympanum is formed by a thin plate of bone, which may 

 be easily broken through so as to obtain a view of the tympanic cavity from 

 above ; it is situated on the upper surface of the petrous portion of the 

 temporal bone, near the angle of union with the squamous portion, from 

 which in its development it is derived. 



The floor is narrow, in consequence of the outer and inner boundaries 

 being inclined towards each other. 



The outer wall is mainly formed by a thin semitransparent membrane 

 mernbrana tympani, which closes the inner end of the external auditory 

 meatus ; and, to a small extent, by bone. Immediately in front of the ring 

 of bone into which the membrana tympani is inserted, is the inner extremity 

 of the fissure of G laser, which gives passage to the laxator tympani muscle, 

 and attachment to the processus gracilis of the malleus. Close to the back of 

 this fissure is the opening of a small canal (named by Cruveilhier the canal 

 of Huguier), through which the chorda tympani nerve usually escapes from 

 the cavity of the tympanum and the skull. 



Fig. 500. MEMBRANA TYMPANI Ficr 500 



AS SEEN FROM THE OUTER 

 AND INNER SIDE. 



A, the outer surface ; B, the 

 inner ; in the latter the small 

 bones are seen adherent to the 

 membrane and adjacent parts of 

 the temporal bone ; in A,- the 

 shaded part indicates the small 

 bones as partially seen through 

 the membrane ; 1, membrana 

 tympani ; 2, malleus ; 3, stapes ; 

 4, incus. 



The membrana tympani is a nearly circular disc, slightly concave on its 

 outer surface. It is inserted into the groove already noticed at the end of 

 the meatus externus, and so obliquely that the membrane inclines towards 

 the anterior and lower part of the canal at an angle of about 45. The 

 handle of the malleus, one of the small bones of the tympanum, descends 

 between the middle and inner layers of the membrana tympani to a little 

 below the centre, where it is firmly fixed ; and, as the direction of this 

 process of the bone is slightly inwards, the outer surface of the mem- 

 brane is thereby rendered concave, being held inwards in the shape of a 

 shallow cone. 



Though very thin, the membrana tympani is composed of three distinct 

 structures. A prolongation of the skin of the external meatus forms the 

 outer layer ; the mucous membrane lining the cavity of the tympanum 

 furnishes an inner layer ; and between those two is the proper substance of 

 the membrane, made up of fine fibrous and elastic tissues with vessels and 

 nerves. The greater number of the fibres radiate from near the centre at 

 the attachment of the handle of the malleus ; but close to the circumfer- 

 ence are some circular fibres, which form a dense, almost ligamentous ring. 



The inner watt of the tympanum, which separates it from the internal ear, 

 is very uneven, presenting several elevations and foramina. Near its upper 

 part is an ovoid, or nearly kidney-shaped opening fenestra ovalis, which 

 leads into the cavity of the vestibule. This opening, the long diameter of 



