SPECIAL PARTS IN TYMPANUM. 



673 



canals which lie on the outer side of the passage for the vessel : the upper 

 (A) contains the tensor tympani muscle ; and the lower (i) is the Eusta- 

 chian tube. Between the two canals is a thin osseous lamina, which is 

 hollowed above and dilated at the inner end, and is named processus 

 cochleariformis. 



Fie. 239. 



Promontory. 



Fenestra ovalis. 



Fenestra rotunda. 



Nerve grooves on the promon- 

 tory. 



Pyramid. 



Prominence of aqueduct of 

 Fallopius. 



Openings of mastoid cells. 



Canal of tensor tympani. 



Osseous part of Eustachian 

 tube. 



Internal carotid artery. 



Facial nerve. 



VIEW OP THE INNER WALL OF THE TYMPANUM ENLARGED (Bowman). 



Some parts that have been referred to above, viz., the membrana tym- 

 pani, the Eustachian tube, and the secondary tympanic membrane, require 

 separate notice. 



The membrana tympana (fig. 240, a) is a thin translucent stratum be- 

 tween the meatus auditorius and the cavity of the tympanum. It measures 

 one-third of an inch across, is oval in form, and is attached by its circum- 

 ference to a groove at the inner end of the auditory passage ; but in the 

 foetus it is fitted into a separate osseous ring, the tympanic bone. The 

 membrane is placed very obliquely so that it forms with the floor of the 

 meatus an angle of 45 degrees, and the outer surface is directed down- 

 wards. Towards the auditory canal the surface is concave ; but in the 

 tympanum it is convex, and attached to its upper half is the handle of 

 the malleus (b) one of the ossicles. 



Structure. The membrane is formed of three strata external, internal, 

 and middle. The outer one is continuous with the integuments of the 

 meatus auditorius ; and the inner is derived from the mucous membrane 

 of the tympanum. The middle layer is formed of white and yellow fibrous 

 tissues, and is fixed to the groove in the bone. From its centre, where it 

 is connected with the handle of the malleus, fibres radiate towards the 

 circumference ; and near the margin, at the inner surface, lies a band of 

 stronger circular fibres (fig. 240, c), which bridges across the notch at 

 the upper part of the tympanic bone. 



The Eustachian tube (fig. 238, e) is the channel through which the tym- 

 panic cavity communicates with the fauces. It is about an inch and a half 

 in length, and is directed downwards and inwards to the pharynx. Like 

 the meatus auditorius, it is partly osseous and partly cartilaginous in 

 texture. 



The osseous part is rather more than half an inch in length, and is nar- 

 rowed at the middle. Its course in the temporal bone is along the angle 

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