The space enclosed by a tubular structure or hollow organ (e.g. the gastrointestinal tract or urinary bladder).
The SI unit of luminous flux (1 lumen [1m] = the amount of light emitted per second in a unit solid angle of 1 steradian by a 1-candela point source).
The patient lies on his/her left side with the back on the edge of the bed and the hips and knees flexed as shown in this diagram. The needle is inserted through the skin between the processes of the third and fourth lumbar vertebrae. The needle is then passed through the spinal ligaments and the dura into the subarachnoid space which contains the spinal fluid. A sample of fluid is slowly withdrawn.