Inability to void may be due to an obstruction somewhere in the urinary outlet or to muscular weakness of the bladder. In not a few instances, patients have come under my care who have undergone an operation for a supposed prostatic obstruction, whereas in reality the condition had been due to a partial paralysis of the bladder muscle. In order to aid the diagnosis in these obscure cases and to study the bladder function with more precision, I have devised an instrument, which is represented in the accompanying illustration. DESCRIPTION OF INSTRUMENT The instrument consists of a catheter (A), a two-way cock with three openings (B), a wide-mouthed 2-ounce bottle with rubber cork (C), an ordinanry blood pressure gage (D), and a piece of rubber tubing,(E) for the purpose of introducing fluid into the bladder or letting it out. Connections are made with rubber and