In typical cases no great difficulty will be experienced in diagnosing between brain abscess, meningitis, and lateral sinus thrombosis. In many cases, however, two or more of these conditions may be co-existent. Thus, both brain abscess and lateral sinus thrombosis are often complicated by the presence of a meningeal infection. Again, the undue prominence of abdominal, cardiac, or pulmonary symptoms--more especially in cases of lateral sinus thrombosis--demand the full consideration of enteric fever, endocarditis, and pneumonia. Thus, in a case recently under my care at the hospital, and in which I had the benefit of the wide experience of my Aural colleague, Mr. West, operative procedures were carried out on the mastoid region, the patient dying some days later from enteric fever. Again, the question of differential diagnosis between tuberculous meningitis, brain tumour and brain abscess is always cropping up. Thus, a short time ago, I explored both temporo-sphenoidal lobes in a boy suffering from double otitis media and presenting well-marked general cerebral symptoms, only to find that he was a subject of tuberculous meningitis. The diagnosis is often difficult, and no trouble must be spared in the complete investigation of the case. _Lumbar puncture_ should invariably be carried out, the fluid being examined both cytologically and bacteriologically. In all cases of doubt one would be wise to call in further advice before undertaking exploratory operation. =Cavernous sinus thrombosis.= Thrombosis of the cavernous sinus arises either as an osteo-phlebitis or as a thrombo-phlebitis. In the former case, the thrombus is dependent on disease of the bones related to the sinus--arising more especially as the result of prolonged sphenoidal sinusitis--whilst, in the latter instance, the sinus becomes infected through one of the many vascular communications connecting it with other neighbouring regions. Thus the radicles of the ophthalmic vein, by means of their communication with orbital, ethmoidal, and upper nasal vessels, and their connexion at the inner canthus of the eye with radicles of the angular vein, afford the most ready means of infection. Again, infection may spread from the auditory region along the superior petrosal sinus, and from the tonsillar and pharyngeal regions along the various emissary veins connecting the cavernous sinus on the one hand with the deep cervical vascular system on the other. The two cavernous sinuses intercommunicate by means of the circular sinus, and consequently thrombosis of the one sinus is liable to involve its fellow. According to Macewen,[57] bilateral trouble occurs in 50 per cent. cases, the infection often spreading from one sinus to the other within twenty-four to thirty-six hours. Thrombosis of the cavernous sinus is often associated with meningeal infection or brain-abscess.