=Pachymeningitis externa.= Pachymeningitis externa is almost necessarily dependent on disease of the middle ear, on suppuration in the accessory sinuses of the nose, and on infected fractures of the vault and base. The inflammatory changes primarily involve the external aspect of the dura mater, the external surface of which is injected, roughened with fibrinous exudate, and, in the more chronic cases, covered with granulations. In itself the process seldom gives rise to symptoms, but so soon as the inner surface of the dura shares in the affection (meningitis), or the infection spreads to the venous sinuses (sinus thrombosis), or to the brain (brain abscess), definite symptoms arise pointing to the nature and character of the change. =Pachymeningitis interna and pachymeningitis interna hæmorrhagica.= As the direct result of alcoholic excess, of syphilitic infection, and of trauma, the inner surface of the dura mater loses its shiny appearance, becomes opaque and thickened, especially in the region of the falx cerebri and over the convexity of the brain. The sodden and œdematous condition of the dura gives rise to a fibrinous and membranous exudate, terminating in the formation of false membranes--one of the factors in the formation of so-called ‘arachnoid cysts’ (see p. 203). These membranes are seen to be attached to the dura mater by means of newly-formed vessels, some of which may rupture, giving origin to blood exudate sufficing to impart to the membranes a brownish-red coloration. In the more marked cases, especially in those of a recurrent nature, the membranes partake of a lamellated type. Occasionally, the hæmorrhages are more excessive, giving rise to the formation of subdural hæmatomata from the partial absorption of which the various types of pachymeningitis hæmorrhagica arise. These rarer forms of meningitis seldom give rise to such definite symptoms as to allow the surgeon to carry out surgical procedures in their earlier stages. Later on, by reason of the development of cystic accumulations of fluid or other complication, operation may be carried out with a reasonable degree of success. The operative technique adapted to the individual case is discussed under appropriate headings--Jacksonian epilepsy, meningeal cysts, &c. It is possible that some criticism may be aroused with regard to the non-inclusion of tuberculous meningitis. Insomuch, however, as I have operated on a considerable number of cases of tuberculous meningitis--some intentionally from a decompression point of view, others by reason of doubtful diagnosis--and have never brought about more than some temporary alleviation, I have come to regard such cases as beyond the realms of surgery. [Illustration: FIG. 83. A FRACTURE OF THE ANTERIOR FOSSA WHICH WAS FOLLOWED BY THE DEVELOPMENT OF MENINGITIS.] =Acute lepto-meningitis.= This condition will be considered under the more familiar, though less correct, designation--_meningitis_.