The appearance of gas shadows in joints under certain conditions is well known. Young children whose laxity of the joint ligaments allows wide separation of the articular surfaces are especially prone to accumulate gas in the joint space when the joint is placed under strain. The occurrence of gas in the joint under these conditions may be regarded as physiological, but, equally, under certain pathological conditions gas may appear in joint spaces. Osteo-arthritic joints are especially prone to show gas in the joint space, but even in such cases gas does not appear unless strain is put on the joint. The large joints of the extremities are the most frequent sites for gas formation; gas formation in intervertebral disc spaces occurs much less frequently, but when it does it is always pathological and indicates the presence of an abnormal space in the intervertebral disc. The origin of gas in joints is to a large extent speculative. Most authorities are agreed that the gas comes from that held in solution in...