ORGAN OF HEARING. 



545 



The tympanic ring lies immediately behind 

 the glenoid cavity of the temporal bone, from 

 which its anterior part is separated by a fissure. 

 The middle part of this fissure, together with a 

 line indicating the whole, remains permanent 

 in the adult, and is known by the name of 

 fissure of Glasser. Its internal orifice is, in the 

 adult as it is in the young bone, in the outer 

 wall of the tympanum, anteriorly of, and close 

 to the groove for the reception of the membrana 

 tympani. The fissure of Glasser gives passage 

 to the ligament, or so-called great external 

 muscle of the malleus, which is inserted into 

 the processus gracilis of the malleus. The 

 chorda tympani does not actually pass through 

 the fissure of Glasser as commonly described, 

 but as M. Iluguier* has shown, through a par- 

 ticular canal, extremely narrow and about half 

 an inch long, which runs in the line of the 

 fissure, and opens at the re-entering angle be- 

 tween the squamous and petrous portions of 

 the temporal bone. 



Membrane of the tympanum, (membrana tym- 

 pani, Fr. la membrane du tympan ou du tam- 

 bour,) Germ, das Trommel felt oder Pauken- 

 J'ell. A proper membrana tympani exists only 

 in birds and mammifera. In reptiles there is a 

 very imperfect representation of one. In birds 

 the membrana tympani is convex externally, in 

 the mammifera, on the contrary, it is concave. 

 The convexity externally in birds forms a very 

 important distinguishing character of the class. 

 In the cetaceous mammifera the membrana tym- 

 pani is thick, and presents a prolongation like 

 the tube of a funnel into the cavity of the tym- 

 panum. 



The membrana tympani is situated at the 

 bottom of the external auditory passage, be- 

 tween which and the cavity of the tympanum 

 it is interposed like a partition. It is a thin, 

 semi-transparent, glistening, dry-looking mem- 

 brane. Its shape is an oval, truncated at one 

 extremity, the upper. Rather more than the 

 upper half of its vertical diameter is traversed 

 by the handle of the malleus, which, when the 

 membrane is examined on the living subject by 

 means of the speculum auris, appears directed 

 from above downwards and backwards. 



The longest diameter of the membrana tym- 

 pani, which is directed from above downwards, 

 and from behind forwards, is about eight- 

 twentieths of an inch, and its shortest, that from 

 behind forwards, somewhat less than seven 

 twentieths of an inch. It is fixed by its cir- 

 cumference in the circular groove already men- 

 tioned, at the inner orifice of the osseous part 

 of the external auditory passage, or in the foetus, 

 the tympanic ring; and as in the adult the ori- 

 fice is cut obliquely from behind forwards, 

 from above downwards, and from without in- 

 wards, so is the direction of the membrane. 

 Hence it forms, with the upper and posterior 

 wall of the auditory passage, an obtuse angle, 

 and with the lower and anterior wall, an acute 

 angle. 



Figure 249 represents the adult membrana 



* Cruveilhier, Anatomie Descriptive, tome iii. 

 p. 508. 



VOL. II. 



tympani of the right side ; a. as seen from the 

 auditory passage ; b. as seen from the tympa- 

 num. Its shape, size, the mode in which the 

 malleus is connected with it, and the cartila- 

 ginous ring which forms its circumference, are 

 sufficiently well shown. 



Fig. 249. 



The membrana tympani does not present 

 plane surfaces. On the contrary its centre is 

 drawn inwards, so that it is concave externally, 

 and convex internally. This disposition of the 

 membrana tympani depends on its connexion 

 with the handle of the malleus. The latter being 

 fixed in its whole length to considerably more 

 than the upper half of the vertical diameter of 

 the former, and having an inward direction in- 

 feriorly, the membrana tympani is, as it were, 

 drawn inwards to it, hence the concavity ex- 

 ternally. 



As regards the composition of the membrana 

 tympani, it consists of a proper membrane and 

 two borrowed layers, one of which, covering 

 the external surface of the proper membrane, is 

 a delicate continuation, in the form of a blind 

 end, of the lining of the auditory passage, and 

 the other, situated on the inner surface, is 

 a continuation of the delicate membrane which 

 gives a lining generally to the cavity of the 

 tympanum. The latter adheres very closely to 

 the proper membrane, the other not so inti- 

 mately, as it readily separates from it by putre- 

 faction, and can be drawn out along with the 

 rest of the epidermis of the external auditory 

 passage in a cul-de-sac. 



Structure of the proper membrane. The pro- 

 per membrane can be divided into two layers, 

 an outer thin one, consisting of radiating fibres, 

 and an inner thicker layer, which is less dis- 

 tinctly fibrous, though when torn it does indi- 

 cate a fibrous disposition, and that in a direc- 

 tion opposite to the former. The radiating 

 fibres run from its circumference towards the 

 centre, to be fixed to the handle of the malleus 

 along its whole extent. Towards the centre 

 they become stronger, and being, of course, 

 more aggregated, the layer which they compose 

 is thicker and more compact in the centre than 

 towards its circumference. The fibres which 

 cross the radiating ones are also more aggre- 

 gated at the centre. They run paiallel with 

 the handle of the malleus, and turn round 

 its extremity. At the circumference of the 

 proper membrane, there is a thick firm ligamen- 

 tous or cartilaginous ring, (fig. 249,) which 

 is fixed in the groove of the bone. This liga- 

 mentous rinsr appears to be formed by an ag- 

 gregation of the circular fibres interwoven with 

 the peripheral extremities of the radiating 

 ones. The pa;t of the membrana tympani 

 midway between its centre and circumference 

 is the thinnest. 



The radiating fibres have been supposed to 

 2 o 



