A single small aperture connecting each lateral ventricle with the dorsocephalic portion of the third ventricle provides the sole means of escape for cerebro-spinal fluid, and consequently it follows that any interference with its patency will seriously retard the outward flow of ventricular fluid. Complete, or even partial, closure of the foramen leads to a rapid increase in the volume and pressure of imprisoned ventricular fluid so that a condition of internal hydrocephalus is soon established. Usually both foramina are occluded, the hydrocephalus being therefore bilateral and the ventricles more or less symmetrically enlarged. In rare instances, however, only one foramen is obstructed, in which case the dilatation will, of course, be confined to the ipsilateral ventricle. Unilateral hydrocephalus of this obstructive type is commonly caused by inflammatory conditions in the neighbourhood of the foramen, or by pedunculated tumours attached to the choroid plexus which enjoy a degree of movement sufficient to permit intermittent or permanent blockage of the foramen of Monro. It is, however, possible to find examples of unilateral hydrocephalus in which the outflow of cerebro-spinal fluid through the foramen of Monro is unimpeded, and in these the cause of the ventricular dilatation is to be sought in some diseased condition of the cerebral wall which has become so weakened as to be unable to resist even normal ventricular pressure. Such expansion may be either limited to one part of the lateral ventricle, or general, involving the entire body with its horns.