This classification possibly includes all the various grades and degrees of internal hydrocephalus, but for all practical purposes the cases may be grouped into two classes, congenital and acquired, both of which may be either acute or chronic. =Congenital internal hydrocephalus.= The condition may be recognized soon after the birth of the child, or the enlargement of the head may only become apparent some weeks or months later. The slow development and the insidious nature of the enlargement, as observed in many cases, may lead to some confusion between the late congenital and the early acquired varieties. However, the absence of any symptoms pointing to meningeal inflammation between the time of the birth of the child and the hydrocephalic development usually allows of correct classification. It is doubtful whether congenital internal hydrocephalus can be ascribed to malformation of the inter-ventricular channels and occlusion of the passages by means of which the cerebro-spinal fluid escapes into the cerebral subarachnoid space, or whether the development is dependent on hypersecretion from the ependyma and choroid plexuses. Virchow showed that occasionally there was an actual formation of small grey-red masses, about the size of a hemp-seed or cherry, in the walls of the ventricles, but other authorities deny the existence of such changes, and consider that the hydrocephalic condition is entirely dependent on congenital malformation. It would, however, seem more probable that we have to deal with two distinct varieties of congenital internal hydrocephalus, one resulting from intra-uterine ependymal inflammation (? syphilitic), the other dependent on congenital malformations, especially in the region of the fourth ventricle, where the foramina of Majendie, Key, and Retzius are regarded as permitting the outward escape of the fluid secreted from the lining membrane and choroid plexuses of the ventricles. =Acquired hydrocephalus.= Acquired hydrocephalus, whether acute or chronic, presents certain antecedents or associations which enable us to have a more clear idea as to the pathological conditions present. In the majority of cases it is secondary to basic meningitis which, whether tuberculous or not, results in matting of membranes and in the development of adhesions. The normal flow of cerebro-spinal fluid from the ventricular to the cerebral subarachnoid spaces is thus impeded. Similar interference to the flow of cerebro-spinal fluid may be caused by the growth of a tumour, especially those which originate in the subtentorial region. =Progress of the case.= Whether the progress of the case be acute or chronic, the ultimate results are much the same. The fluid in the ventricular spaces may be increased up to 1,000 c.c. or more, pressure effects being exerted on the surrounding parts, with the following results:--