Purulent collections under the cranium, between the bone and dura mater, are not of very frequent occurrence, when symptoms are well watched and treatment properly conducted. But these collections certainly may and do occur, and usually at a considerable period after the accident: many such cases are related by the older authors. Their attendant symptoms are materially different from those of extravasated blood; in the latter case, all the symptoms of compression ensue immediately after the effusion has occurred, and that is generally very shortly after the injury. But matter is not formed till after a considerable period has elapsed; it is not attended with symptoms of compression suddenly supervening, but is preceded by restlessness or febrile excitement; and in the later stages only of the affection do the symptoms of cerebral compression manifest themselves. By the external injury, those bloodvessels by which the dura mater is attached to the skull, and by which it communicates with the pericranium and more external parts, are lacerated, or otherwise materially injured, inflammatory action is excited in the connecting medium, unhealthy suppuration ensues, and by the accumulation of matter, the membrane is completely separated from the cranium, and generally participates in the morbid action. It may ultimately slough and give way, and the matter will then be effused internally. A similar process goes on in regard to the bone and its pericranium, a tumour forms externally, and the bone, being deprived of its supply of blood, necessarily dies, either in part, or throughout its whole thickness. When an external wound exists, the altered appearance of the bone, with the sloughy state of the detached pericranium, gives evident warning of the mischief which is proceeding internally. The general symptoms of suppuration are the same, whether the collection forms in the substance of the brain, or on its surface. Perhaps the symptoms are not so severe, nor the collection so speedily fatal, when in the substance of the brain, as when situated immediately under the bone, or at the base of the cranium. The external marks already mentioned, are generally indicative of the site of such internal collection, but not uniformly. Formation of matter in the diploe of the skull, in consequence of external injury, is of rare occurrence; and when it does occur, somewhat similar symptoms and appearances ultimately ensue as when the suppuration commences between the bone and dura mater.