These venous channels are somewhat frequently the seat of thrombosis. The coagulation may have its starting point in an inflammation propagated from a neighbouring structure, in which case it is usually a septic thrombosis. The most frequent origin of such inflammations is disease in the ear, but it may follow on injury to the head, inflammations of the skin of* the face and scalp, especially erysipelas, and of the bones. In all these cases the thrombosis usually has its starting point about the base of the skull, and especially in the lateral and petrosal sinuses.

There is, however, another class of cases in which the thrombosis has a more obscure origin, and seems sometimes even spontaneous. The blood simply coagulates in the sinus, and it is usually in the longitudinal sinus that the coagulation begins. In most of these cases the person is in a state of debility, and the thrombus may be regarded as Marasmic in its origin. But there are cases in which there is no obvious weakness of the heart, and the coagulation has no apparent cause. The localization of the thrombosis suggests that it has been occasioned by stagnation of blood. The sinuses are rigid tubes incapable of narrowing when the circulation is slow, and they are intersected by bands of connective tissue. The longitudinal sinus also is so situated that, at its middle part at least, the blood passing from the cerebral veins flows upwards to it against the force of gravitation (see p. 97), and at right angles to the current in the sinus.

The Effects of thrombosis of the sinuses will vary according to the cause. If due to the propagation of inflammation from neigbouring structures, then a suppurative phlebitis may result, with Meningitis, and even Abscess of the brain. There are, however, cases of abscess of the brain without meningitis, in which the inflammation seems to have extended along the veins either in their interior or in their sheaths.

In marasmic thrombosis the results are those of passive hyperemia. The veins which open into the sinus are greatly engorged, and the thrombosis may extend into them. It is here chiefly the longitudinal sinus that is concerned, and the veins which open into it are those of the cerebral hemispheres. These may stand out as prominent wormlike cords filled with dark coagula. In most cases the blood finds its way by other routes, and there may even, after a time, be a re-establishment of the circulation in the sinus. Sometimes, however, the obstruction in the veins is such as to lead to Haemorrhage. This is usually in the form of numerous capillary haemorrhages, but sometimes there is along with this a large effusion of blood in the substance of the brain and under the soft membranes. The seat of such haemorrhages is usually the superficial and upper parts of the hemispheres, and they are frequently multiple.

Gowers alleges that a primary thrombosis of cortical veins without sinus thrombosis not infrequently occurs, and is a not uncommon cause of hemiplegic attacks in young children, ushered in by convulsions which may be unilateral.