The trephine is applied to the bone, the surgeon aiming at the angle between the anterior and posterior branches of the middle meningeal artery. The disk is removed, the dura separated from the bone, and the craniectomy forceps called into requisition, the bone entering into the formation of the temporal fossa being freely cut away, more especially in the _downward_, forward, and backward directions--in other words, in the general line of the temporo-sphenoidal lobe. In the upward direction the surgeon must be more guarded, more especially when the operation is being conducted on the left side of the head. Broca’s area of motor speech must be avoided, for fear of its inclusion in any hernial protrusion that may ensue. The bulging and probably discoloured dura mater is freely incised, preferably in a crucial manner, though the exact line of such incision is of little importance so long as it is free in character. Needless to say, all meningeal vessels that cross the lines proposed for dural section must first be ligatured (by underrunning with a fully curved needle) on either side of those lines. The four dural flaps are turned aside and the antero-external aspect of the temporo-sphenoidal pole exposed. A small drainage--or drain of rubber tissue--can now be inserted along the floor of the middle fossa skull in the immediate vicinity of any lacerated brain or blood-clot, the drain lying between the dura and the brain, and anchored to the dura mater or muscle by a fine catgut suture. The four dural flaps are allowed to remain loose over the surface of the brain, whilst the temporal muscle is sewn across from side to side with a few catgut sutures, room being allowed at the lower angle for the emergence of the drain. The temporal fascia is replaced and carefully united to its upper cut margin, the drain being brought out through the fascia and through a puncture hole made at the most convenient part of the scalp-flap. The scalp-flap is approximated with numerous interrupted silk or salmon-gut sutures. The tube is allowed to remain for thirty-six hours or more, according to the condition of the patient. The operation may be carried out on one or on both sides of the skull; if on the one side only, on that side at which laceration of the brain is probably existent. In connexion with this it is necessary to state that laceration by contre-coup is more common than direct brain-injury. Thus, if the blow be inflicted on the right parieto-occipital region, the operation should be conducted in the left temporal region. In the absence of all localizing features the decompression should be carried out on the _right_ side of the head, in order to avoid all possibility of including, in the hernial protrusion that may result, the motor speech area of Broca.