Hydrocephalus may occur, either as an idiopathic or symptomatic affection. As the first, it may arise, where there exists a predisposition in the brain, from various injuries inflicted on the head by slight blows;--from all the general causes of inflammation--from the sudden drying up of long established discharges--the sudden repulsion of cutaneous eruptions, or the imperfect evolution of that or other sanative actions of the system, at the close of some febrile diseases, usually denominated defect of crisis. When, on the other hand, the disease is symptomatic, it may arise from a particular cause seated within the head, or in some distant part of the body. The former variety is not common among children, and when it does occur, it is the result of some chronic disease, as a tumour or a thickened state of the arachnoid or other membranes of the brain, resulting from a former inflammation. "Sometimes, adult patients wholly recover from chronic or sub-acute inflammation, which induced the structural disease, and this last becomes, at some future period, the occasional cause of the hydropic one." At other times, the chronic inflammation continues, and finally extends to the serous membrane, giving rise to the effusion. "The most usual cause of the disease, however, particularly in children, is an irritation which is sympathetically communicated to the brain, from a disturbance in the chylopoietic organs; and particularly from a functional disorder of the liver. The cerebral disorder, to which a derangement in the digestive functions thus gives rise, is only one of those numerous effects which arise out of sympathies, subsisting between these organs and different parts of the system. In many cases, the same sympathetic irritation is successively and variously directed to different parts of the system. It will thus leave one organ or part, and suddenly move to another; and through the operation of causes, which are not always obvious, but which have a relation to some particular predisposition, inherent or acquired. In this way, an irritation may occasion an eruption upon the skin, and thence be translated to the bronchial lining, producing a cough; and next perhaps, to the serous tissue of the brain, exciting there a turgescent or congestive state of the cerebral vessels, by which symptoms are produced, through the pressure of the congestive vessels, that simulate those of hydrocephalus; or the true disease is brought on by an arterial re-action, ensuing upon the congestion, which is resolved by a serous effusion."