As the result of chronic middle ear disease, the mucous lining of the middle ear and its accessory cavities becomes destroyed, the antrum filled with cholesteatomata, and the middle and external ears with granulations. The discharge of pus, previously free, is obstructed, partial or complete blockage occurring. The destruction of the mucous lining allows of invasion of the surrounding bone, the veins become thrombosed and filled with bacteria, and the cancellous spaces blocked with granulations. Further erosion of the bone results, both in the upward direction towards the tegmen tympani and in the backward towards the lateral sinus groove and cerebellum. The veins of the tegmen communicate freely with those of the temporo-sphenoidal lobe, whilst those ramifying in the mastoid region either communicate with the lateral sinus itself or with the anterior cerebellar venous system. Infection may therefore spread (1) _upwards_ to the temporo-sphenoidal lobe, or (2) _backwards_ to the lateral sinus and cerebellum. In the former case, meningitis or temporo-sphenoidal abscess develops: in the latter instance, meningitis, lateral sinus thrombosis, or cerebellar abscess. For the formation of a brain abscess it is, of course, essential that the brain membranes overlying the main site of osseous erosion should be sealed off in such a manner as to prevent a general infection of the meningeal region, the dura becoming adherent to the eroded tegmen, &c. It is œdematous and throws out granulations, both on its parietal and visceral aspects. The parietal granulations aid in the further erosion of the bone, whilst the visceral may, according to Macewen,[54] even indent the brain. By means of thrombosed veins, perivascular lymphatics, and minute arterioles, a channel of infection is now opened up between the site of osseous erosion and the temporo-sphenoidal and cerebellar lobes. [Illustration: FIG. 74A. DIAGRAM TO SHOW EXTENSION OF DISEASE FROM TYMPANIC CAVITY, IN MIDDLE EAR SUPPURATION. 1, Perforation through tympanic membrane; 3, Perforation through Shrapnell’s membrane; 4, Fistula through outer wall of attic (roof of external meatus); 5_a_, Extradural abscess; 5_b_, Meningitis; 5_c_, Temporo-sphenoidal abscess; 6, Bulb of jugular (thrombosis); 11, Internal ear; 12, 12_a_, 13, Route of infection through internal ear giving rise to extradural abscess, meningitis, and cerebellar abscess. (After Hunter Tod.)]