At the bedside there is often difficulty in discriminating between the various pyogenic intra-cranial complications, because many of the symptoms are common to all the members of this group, and because more than one condition is frequently present. Thus a localised meningitis spreading to the brain may set up a cerebral abscess; a sinus phlebitis may give rise to a purulent lepto-meningitis; or a cerebral abscess bursting into the sub-arachnoid space may produce meningitis. MENINGITIS #Pachymeningitis.#--This term is applied when the infection involves the dura mater--a condition which is usually due to the spread of infection from a localised osseous lesion, such as erosion of the tegmen tympani in chronic suppuration of the middle ear, of the wall of the sigmoid groove in mastoid disease, or of the posterior wall of the frontal sinus in suppuration of that cavity. It also occurs in relation to septic lesions of the cranial bones such as a broken-down gumma, after operations on the cranial bones, and in cases of compound fracture attended with a mild degree of infection and with imperfect drainage. In contusion of the skull without an external wound, the infection may take place through the blood stream. The layer of the dura in contact with the affected portion of bone is inflamed, thickened, and covered with a layer of granulations--_external pachymeningitis_--and between it and the bone there is an effusion of fluid. Up to this point the process is largely protective in its effects, and gives rise to no symptoms, beyond perhaps some pain in the head. In the majority of cases, however, suppuration occurs between the dura and the bone--_suppurative pachymeningitis_--and leads to the formation of an _extra-dural abscess_ (Fig. 192). When this happens in association with disease in the middle ear or frontal sinus, it is attended with severe headache referred to the seat of the abscess, a sudden rise of temperature preceded by shivering, and other evidence of the absorption of toxins. Over the situation of the abscess, the scalp becomes swollen and œdematous--a condition which Percival Pott, in 1760, first observed to be characteristic of extra-dural suppuration, hence the name, _Pott's puffy tumour_, applied to it (Fig. 193). Under these circumstances the abscess is seldom of sufficient size to cause a marked increase in the intra-cranial tension, or to give rise to localised cerebral symptoms by pressing on the brain. [Illustration: FIG. 192.--Diagram of Extra-Dural Abscess.] [Illustration: FIG. 193.--Pott's Puffy Tumour in case of extra-dural abscess following compound fracture of orbital margin; infected with road-dust; operation; recovery. At the time of the photograph the man was unconscious.]