We have seen that these vessels run in the sulci and fissures of the brain, involved in the meshes of the pia-arachnoid, and it might be supposed that their rupture would give rise to meningeal rather than cerebral haemorrhage, and so it is usually stated. But this is not the case. The blood nearly always finds its way into the brain substance, where it is found usually in much larger quantity than in the meninges, so that the case has much more the characters of cerebral than of meningeal haemorrhage. There is usually some blood in the meninges occupying the sulci between the convolutions, and it may even be considerable, extending to the base and perhaps covering the optic commissure; but usually the amount is small, and, on account of the large cerebral haemorrhage, is apt to be overlooked. It is very rare indeed for the blood to escape to the surface and appear in the cavity.of the dura mater. Any appearance of blood in the meninges, however, should at once direct attention to the larger cerebral arteries as the probable source of the haemorrhage.

The explanation of the extension of the blood into the brain substance suggests itself when we consider the circumstances of the parts. When rupture of such an artery occurs the blood tears its way around; it passes into the loose connective tissue, and by and* by reaches the surface of the brain. Here the tissue, being soft, tears readily, and the blood rapidly passes inwards. On the other hand the connective tissue on the surface is tough and divided by numerous bands and partitions, and the blood will tear it with difficulty. The blood may work its way from space to space in the connective tissue, but this takes time, and probably needs considerable pressure. There is another circumstance which probably has to do with the blood so constantly finding its way into the brain substance. As we shall see afterwards, a large number of the cases of haemorrhage in this situation are from rupture of aneurysms. Now an aneurysm will probably project more readily towards the surface of the brain where the substance is soft, than in other direcfions, and when such an aneurysm ruptures it may do so directly into the brain.

The Cause of the rupture of these larger arteries is to be sought for in disease of their walls and increase of the blood-pressure.

The commonest cause is Aneurysm of these arteries, which is of remarkably frequent occurrence. The great majority of the cases of cerebral haemorrhage occurring before the age of fifty years is due to the rupture of aneurysms of these larger arteries. The aneurysms are mostly thin-walled and therefore prone to rupture. Their most frequent seat is on the middle cerebral artery or one of its branches in the fissure of Sylvius, but they may occur on any of the arteries of the brain and are not infrequently multiple (see Fig. 327).

Fig. 327. - Aneurysms of larger cerebral arteries. A large one at the bifurcation of the left middle cerebral. There is an aperture at its summit, from which fatal haemorrhage occurred. A small one near the origin of right anterior cerebral.

The frequent occurrence of these aneurysms and their serious import suggests an inquiry into the causes of their formation. In the first place the arteries are thin-walled and are placed in a loose tissue, so that they very readily undergo dilatation. Any local injury to the wall may be the starting point of the dilatation. This is often effected by Embolism. In a large proportion of cases the aneurysm is associated with valvular disease of the heart, and an embolus imperfectly obstructing an artery, especially if it be a cretaceous piece broken off from a valve, may readily injure the wall so as to allow of dilatation. Ponfick has found that in a considerable proportion of cases of acute endocarditis there is embolism, with either fully formed or incipient aneurysms. Another indication of the origin from embolism is that the aneurysm is frequently at a bifurcation. The bifurcation of an artery is a common situation, as in Fig. 327, and more particularly the bifurcation of the internal carotid into middle and anterior cerebral. The aneurysm in that case is almost a bulged out continuation of the carotid, and this suggests an embolus pitched against the projecting angle between the branches. When they have an origin such as this the aneurysms will be specially thin-walled and partake of the characters of false aneurysms. This mode of origin also goes far to explain the greater predominance of these aneurysms in the middle cerebral artery, which, as we have seen, is especially liable to embolism. Another occasional cause of cerebral aneurysm is Atheroma. This disease injures the vessel-wall and produces obstruction, and it may lead to aneurysm on the one hand by weakening the wall, and on the other by locally increasing the blood-pressure on the proximal side of the obstruction. Syphilitic disease is also assigned as a cause of aneurysm.

Atheroma of the larger cerebral arteries is a frequent lesion, and it is sometimes associated with haemorrhage in the substance of the brain. The haemorrhage is not from the larger arteries at present under consideration, but from the nutrient arteries, to the more considerable of which the atheroma may extend.