As soon as the antrum is exposed, the seeker or nerve-protector should be introduced into the aditus, acting as a protector to the underlying external semicircular canal and facial nerve, and the ‘bridge’ of bone, which now intervenes between the antrum and the middle ear, chiselled away, in the manner indicated in Fig. 80. The field of operation is now carefully cleansed, cholesteatomata, bone-debris, and ossicles being gently removed, and--with the aid of a head-lamp--a careful search instituted for fistulous tracts, carious bone, &c. This search must be carried out with the greatest gentleness for fear of inducing further complications. Subsequent procedures vary according to the nature of the conditions found. Thus:-- 1. When the diagnosis is uncertain and when the boundaries of the cavity appear fairly healthy, the operation may terminate at this point, a post-meatal flap being formed and the cavity lightly plugged with gauze. The proximal end of the gauze plug is brought out through the enlarged auditory meatus and the ear-flap sewn back with a few salmon-gut sutures. [Illustration: FIG. 81. EXPLORATION FOR A TEMPORO-SPHENOIDAL ABSCESS. A, Above the tegmen tympani; B, Through the tegmen tympani. Occasionally these methods are combined; the bone between the openings being also removed. (After Hunter Tod.)] 2. When the evidence points to implication of the _middle_ fossa, the scalp incision is prolonged upwards for about 1 inch and the soft parts retracted. The supra-tegmental extra-dural space can be exposed either by chiselling away the osseous roof of antrum and middle ear, or by means of a separate opening above the level of the supramastoid crest. In the event of the discovery of an _extra-dural_ collection of pus, the purulent material is gently wiped away, free drainage supplied, and the wound closed. In both this and the preceding case, the progress of the case during the ensuing twenty-four to forty-eight hours will prove whether adequate measures have been adopted. When the symptoms are suggestive of _brain abscess_, when no extra-dural collection is discovered, and when the dura mater is tense and discoloured, then it becomes necessary to explore the brain. This can be carried out through an opening made in the tegmen or above the level of the supramastoid crest. Which route should be utilized depends on the local conditions. As a general rule, it is advisable to explore through the tegmen when a sinus exists in that situation, and above the supramastoid crest under all other circumstances. [Illustration: FIG. 82. EXPLORATION FOR A CEREBELLAR ABSCESS. A behind, and C in front of the lateral sinus; B, Lateral sinus. (After Hunter Tod.)]