=Treatment.= The mastoid antrum is rapidly exposed and the conditions investigated. The bone is then chiselled away in the backward direction, with the hammer and gouge, so as to expose the lateral sinus. The question then arises as to the condition of the sinus. This is a matter that may require considerable experience. The surgeon should be guided, not so much by exploratory puncture as by the surroundings and general appearance of the sinus. Thus, the absence of bleeding from the mastoid emissary vein during the process of exposure is very significant of sinus thrombosis. Again, whilst the normal sinus pulsates, is of dark blue colour and presents a shining surface, the thrombosed channel may be covered with pale granulations or obscured by fibrinous deposit, it does not pulsate, and appears of a yellow or deep purple colour. Between it and the bone there may be a collection of purulent matter. In the event of doubt, the surrounding regions should be carefully protected with gauze, after which the sinus may be punctured with a needle. The absence of fluid blood is conclusive of thrombosis. [Illustration: FIG. 85A. DIAGRAM TO SHOW THE USUAL POINTS AT WHICH THE LATERAL SINUS IS PRIMARILY INFECTED. A, High up; from the posterior mastoid cells. In this case it may not be necessary to tie the jugular vein. B, Low down; involving the jugular bulb. This necessitates ligature of the vein. (After Hunter Tod.)] [Illustration: FIG. 85B. THE LATERAL SINUS EXPOSED AND OPENED. The lumen of the sinus is obliterated above and below the region of the infected thrombus by plugs of ribbon gauze pressed in between the sinus wall and the overlying bone. In this case it is not necessary to tie the jugular vein. (After Hunter Tod.)] In the event of the surgeon concluding that thrombosis is present, subsequent procedures vary according to the extent of the thrombus. Thus, _when the clot appears to terminate above the jugular bulb_, a strip of gauze is inserted between the bone and the parietal wall of the sinus so as to obliterate the lumen of the sinus on the cardiac side of the clot. The danger of further extension of the clot being thus obviated, the bone is nibbled away in the backward direction till at least half an inch of healthy vessel is exposed on the occipital side of the thrombus. A second gauze plug is then introduced so as to obliterate the sinus lumen in that region also.