[Illustration: FIG. 74B. DIAGRAM TO SHOW SURGICAL ANATOMY FOR OPERATIONS FOR OTITIC INTRACRANIAL LESIONS. 1, Attic; 2, Antrum; 3, Point for opening temporo-sphenoidal abscess (just above and along the tegmen tympani); 4, External semicircular canal; 5, Lateral sinus; also shows area of bone removed in mastoid operation; 6, Bulb of jugular vein; 7, Facial nerve; 8, Point for opening cerebellum behind lateral sinus; 9, Point for opening cerebellum in front of lateral sinus (between sinus behind and external semicircular canal in front). (After Hunter Tod.)] In any case the abscess usually develops in the white substance of the brain, just beneath the grey matter, and in close relation to the primary source of infection. _The wall of the abscess._ In an abscess of rapid development the walls are ragged and but ill-defined from the surrounding brain substance. In the more chronic cases the cavity is shut off from the brain by a capsule of varying density--usually not more than one-sixteenth of an inch in thickness, but occasionally of much greater development (see Fig. 86A). Sometimes--more especially in cerebellar cases--two cavities may be superimposed, the two abscesses communicating with one another by means of a narrow, and often tortuous, channel. Successful drainage of the one cavity may be inadequate to drain its companion. This two-saced abscess will be again alluded to at a later period. _Contents of the abscess._ The pus is usually of a greenish hue, odourless, and acid in reaction. Though often sterile the following bacteria may be cultivated--_staphylococcus pyogenes aureus_ and _albus_, _pneumococcus_, _bacillus pyocyaneus_, and the _streptococcus pyogenes_. The pus is usually of such viscidity that it cannot be evacuated through the lumen of an ordinary aspirating needle. _The size of the abscess._ In size the abscess varies greatly, seldom, however, containing more than 5 or 6 drachms of pus. _The neck of the abscess._ It was first pointed out by Ballance[55] that most brain abscesses secondary to disease of the bones of the skull are ‘mushroom-shaped’, the narrow portion or stem being attached to the dura mater at the original site of infection. The stalked form of abscess is ‘quite comparable, as to its mode of formation, to the superficial cervical abscess connected by a narrow track to a focus of disease beneath the deep fascia’. _Course pursued by an untreated brain abscess._ The abscess enlarges at the expense of the surrounding tissues, either bursting into the ventricles of the brain or coming to the surface, there rupturing and leading to a diffuse meningeal infection. Some few cases have been reported in which the abscess has established a more or less efficient natural means of drainage, either along the ‘stalk’ of the abscess or by means of a new channel to the exterior. In one or two cases, spontaneous recovery has apparently taken place.