AFTER the observation in man that leakage from a small aneurysm induced thrombosis within and around the sac, Dott first attempted operative reinforcement of what he termed, "Nature's attempt at healing."1On April 22, 1931, Dott exposed a congenital cerebral aneurysm at the junction of the anterior and middle cerebral arteries, which promptly ruptured. The hemorrhage was controlled by a pledget of leg muscle, and the arteries and the aneurysm were subsequently surrounded by thin sheets of muscle. This case was reported two years after operation, and at that time the patient was well. In our series of 57 congenital cerebral aneurysms treated by an intracranial operative procedure, 4 have been packed by muscle or with oxidized cellulose, as described by Dott. In two of these cases a recurrent subarachnoid hemorrhage occurred. As a result of this experience in the human being, a number of tissue irritants have been