In the latter case, the dura mater is incised crucially and the bulging brain explored in the manner indicated on p. 266. The mastoid region should first be cleansed and packed with gauze, but, even with such precautions, it is obvious that there is some risk of contaminating membranes and brain in the event of failure to discover the abscess cavity--an argument in favour of exploring through the ‘clean’ squamous region (see p. 264). 3. When the evidence points to implication of the _posterior_ fossa, an incision is carried backwards, from the mid-point of the post-aural incision, for about 2 inches, and the soft parts retracted upwards and downwards. With the chisel (or gouge) and hammer, the bone is freely cut away so as to expose (1) the lateral sinus, and (2) the dura mater below and behind the curve of the sinus. As soon as the sinus is exposed, the dura mater may be separated from the bone and the subsinus region exposed with the aid of craniectomy forceps. The extra-dural space between the posterior aspect of the petrous bone and the lateral sinus region is first inspected, and, in the event of the discovery of a collection of pus, this is gently wiped away and further exploration postponed till the occasion should prove the necessity. When the indications are of such a nature as to demand _exploration of the cerebellum_, the mastoid region is first cleansed and packed with gauze, after which the dura mater is incised either in front or behind the sinus according to the probable situation of the abscess cavity. After the evacuation of the abscess a drainage tube is stitched in position in the manner previously indicated. Difficulties and dangers attendant on the process of trephining for brain abscess. _Failure to find the abscess_ is usually dependent on one of the following causes:-- (_a_) The abscess may be missed if the localizing features are misleading, if the abscess be small, and if the exploring instrument be passed in a faulty direction or to an insufficient depth. (_b_) The abscess may be traversed but not tapped if unsuitable exploring instruments be used--an aspirating syringe, for instance, through which the thick pus will not pass. Even when suitable instruments are used, the attempt to evacuate the abscess may fail if the operator does not carry out what may be called a system of progressive exploration, that is to say, if he does not periodically open the blades of the evacuator (see p. 266). (_c_) The abscess may be encountered but not penetrated if the surgeon has to deal with a chronic abscess, the wall of which is merely pushed aside by the advancing instrument (see chronic abscess of the brain, p. 268). _Complications_ arising during and after the evacuation of the abscess are as follows:-- 1. Leaking into the pia-arachnoid and the development of general meningeal infection. 2. Perforation of the ventricular spaces. 3. Hæmorrhage. 4. Respiratory failure.