Two changes may now occur:--(1) the central portion of the thrombus, having softened into a purulent material, may be limited by the firmer thrombus in front and behind; and (2) the thrombus may extend into the neighbouring and connecting venous channels _downwards_ along the course of the internal jugular vein, _backwards_ along the course of the lateral sinus, _inwards_ along the course of the superior petrosal sinus, _outwards_ along the line of the mastoid emissary vein, _downwards_ through the posterior condyloid foramen, and _inwards_ along meningeal veins. The =symptomatology= may be considered as follows:-- (1) Symptoms dependent on the extension of the thrombus to neighbouring venous and lymphatic channels. (2) Symptoms resulting from toxic absorption or dependent on the transmission of infected material to other parts of the body. 1. _Symptoms dependent on the extension of the thrombus to neighbouring venous and lymphatic channels._ When the thrombus spreads downwards along the course of the _internal jugular vein_, there is swelling and tenderness along the line of the vein. The vessel, though thrombosed in its upper part--perhaps throughout its whole extent--is itself seldom palpable, the cervical swelling usually being dependent on associated lymphadenitis and lymphangitis. In those few cases where the thrombosed vein can be felt, resembling a ‘buried lead pencil’, the thrombus is sometimes of the non-infective type. The combined venous and lymphatic involvement causes œdema of the tissues, pain, and rigidity on attempted movement. The anterior and external jugular veins may become engorged from the extra strain thrown upon them. Subsequently, the inflamed parts may break down and extensive cervical suppuration result. When the thrombus spreads _backwards_ along the course of the _lateral sinus_, the coagulation process may extend as far as the torcula and even further. Evidence as to the nature and extent of the process is not always apparent, though one expects to find some œdema of the overlying scalp tissues. When the process spreads _inwards_ along the course of the _superior petrosal sinus_ there is considerable risk of involvement of the corresponding cavernous sinus, possibly of the opposite sinus also (see p. 288). When the process spreads _outwards_ along the course of the _mastoid emissary vein_, œdema, and dilatation of veins in the post-auricular region are observed, associated with tenderness on palpation. This is most apparent at the upper and posterior border of the mastoid process. When the thrombus spreads _downwards_ along the course of the _posterior condyloid vein_, there may be some œdema and pain on pressure in the upper part of the posterior triangle of the neck, associated with glandular enlargement in the region. These symptoms are dependent on the connexion established by the posterior condyloid vein between the sigmoid sinus and the deep veins of the neck.