HYDROCEPHALUS due to an obstruction or block in the ventricles or subarachnoid spaces is commoner in the infant than is generally supposed. A routine investigation of cases of infantile hydrocephalus will readily satisfy any one on this point. In my experience, the ratio of cases of obstructive hydrocephalus to cases of communicating hydrocephalus is about 2:1. Patients with obstructive hydrocephalus present an obvious challenge to the neurosurgeon, for if the obstruction can be relieved or circuited the intraventricular pressure, with its damage to cerebral tissue and progressive enlargement of the head, may be brought under control. Since there are many causes of obstruction, the same procedure cannot be used or considered for all patients. A frequent cause of obstruction or block in infantile hydrocephalus is the Arnold-Chiari malformation, associated with spina bifida in many patients, especially those with myelocele. In this condition the medulla oblongata is elongated and extends downward