5. Osteomyelitis of the neighbouring diploic tissue. _General meningeal infection_ will bring about a fatal result. Fortunately, the general rise of intracranial pressure and the formation of adhesions between the brain and the overlying membranes tend to prevent its development. _Perforation of the ventricular spaces_ may occur under two conditions: (1) when the abscess is associated with internal hydrocephalus, and (2) when the exploring instrument is passed too far. _Hæmorrhage_ is seldom serious when the operation is carried out with a light hand, all visible vessels being carefully avoided. In the event of its occurrence, it may be controlled by lightly packing the cavity with gauze. _Respiratory failure_ is most liable to occur when the surgeon is operating for cerebellar abscess. In the event of its occurrence the surgeon should complete his trephining with the utmost expedition, thus relieving the intracranial pressure. Under favourable circumstances, the respiratory rhythm is soon restored. In the more serious cases, artificial respiration should be attempted whilst the surgeon carries on his manipulations, opening the skull as rapidly as possible. MENINGITIS Previous to entering into the question of general meningeal infection, some allusion must be made to the rarer forms of meningitis. =Serous meningitis.= Quincke demonstrated in 1895 the existence of a serous form of meningitis, one characterized by a sero-fibrinous exudate. He showed that, just as we have to deal with a serous or purulent pleurisy, so we have to consider the possibility of a serous or purulent form of meningitis. The exudate appears at first sight to differ but slightly from normal cerebro-spinal fluid, being clear, yielding the normal chemical reactions of that fluid, and occupying the subarachnoid region. Later on it becomes slightly turbid, contains a fair quantity of albumen, and is proportionately rich in cellular elements--lymphocytes and polymorphonuclear leucocytes. The membranes are swollen, the vessels dilated, whilst occasionally many minute hæmorrhages may so allow of the escape of red blood-corpuscles as to impart to the exudate a yellowish-red coloration. At a still later stage the exudate becomes flocculent as a result of the deposition of a fibrinous coagulation, a gum-like material forming over the surface of the brain.