=Lateral sinus thrombosis.= Soon after entering on its course across the mastoid process the lateral sinus presents a well-marked S-shaped curve. This sigmoid sinus bulges markedly forwards--especially on the right side--towards the region of the mastoid cells and antrum, so much so that a mere shell of bone intervenes between the sinus on the one hand and the antral region on the other. Indeed, the relations are so intimate that one would expect a more frequent occurrence of lateral sinus thrombosis. Furthermore, the sinus receives numerous venous communications from the mastoid cells, antrum, and other parts of the temporal bone, conspicuous amongst the last-named group of vessels being the mastoid emissary vein which, passing inwards at the upper and posterior border of the mastoid process, connects the posterior auricular and occipital veins with the lateral sinus. From these considerations it is obvious that any acute or chronic infective process originating either in the aural region or in the neighbourhood can readily infect the sinus by direct propagation of organisms along one or more of these inter-communicating vessels (thrombo-phlebitis). The sinus may also become infected in middle-ear disease by the more gradual process of mining and sapping, the osseous barrier between the antrum and sinus being progressively destroyed by the backward progress of the aural disease. The sinus may erect an additional barrier by throwing out granulations (external pachymeningitis) against the invading host, but, in the event of the attack overcoming the defence, sinus thrombosis may result, at first perhaps of a non-infective type but soon becoming definitely septic, the clot softening and disintegrating (osteo-phlebitis). There can be no question that middle-ear disease is responsible for the very great majority of cases of lateral sinus thrombosis, and whether the infection takes place by extension of thrombus along the connecting veins (thrombo-phlebitis), or after destruction of the osseous barrier (osteo-thrombosis), the results are more or less identical. Thrombosis first occurs in that part of the sinus which is in closest relation to the primary cause of the infection, the clot rapidly increasing in size until the lumen of the sinus is entirely obliterated. In its early stages the thrombus is of a chocolate colour, softening at a later date and breaking down into a purulent material. Many varieties of bacteria may be present though the infection is mainly dependent on the presence of the streptococcus pyogenes.