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The effects of puncture of fresh cadaver dura with 20-, 22-, 26-and 29-gauge needfes were observed. A 'tin-lid' phenomenon, manifested with all needle sizes, was capable of sealing the resultant hole. The larger the needle, the larger the hole, while roturion of' the needle bevel 90' to the jibres altered the shape of' the hole. Holes made in thicker parts of the dura tended to retruct more rapidly than those in thinner areas.