OTITIS MEDIA. 463 



It must also be remembered that purulent fluid may find its way 

 into the mastoid antrum, through its foramen of communication 

 with the tympanic cavity. Further, the mastoid cells when 

 developed may also become infected through the antrum, and 

 mastoiditis result. 



The roof of the tympanic cavity is composed of very thin 

 bone, covered above by fairly firmly attached dura mater. The 

 interior of the cranium may be reached by pathogenic micro- 

 organisms from the middle ear in various ways. Carious erosion 

 of bone is probably the commonest method of extension, giving 

 rise to an extra-dural collection of pus, and later on to meningitis. 

 Another route is along lymphatic channels passing between the 

 membranes and the tympanum; or along fibrous trabeculae, 

 stretching between the mucous membrane of the tympanic 

 cavity and the dura mater covering the petrous portion of the 

 temporal bone. Possibly also the bacteria may pass through 

 the petro-squamosal suture in quite young subjects. Also 

 infection may track along the sheaths of the facial and auditory 

 nerves. It is, however, very difficult to explain the exact method 

 by which micro-organisms may cause inflammation within the 

 substance of the cerebellum or the temporo-sphenoidal lobe, 

 without any meningitis being present intervening between the 

 site of the abscess and the source of infection. 



The proximity of the sigmoid portion of the lateral sinus to 

 the mastoid antrum and cells, explains the occurrence of septic 

 thrombosis and subsequent pyaemia. 



Occasionally pus from the middle ear may track downwards 

 along the side of the Eustachian tube, and give rise to a lateral 

 retro-pharyngeal abscess. 



The course of the facial nerve from the internal auditory 

 meatus through the aqueductus Fallopii to the stylo-mastoid 

 foramen, lying in close relationship with the inner angle of the 

 roof of the tympanum, and subsequently along the posterior wall 

 of this cavity, indicates the reason why otitis media may be 

 accompanied by facial paralysis. 



Pus in the mastoid antrum is placed in o cavity which can 



