This dislocation is usually produced by extreme dorsiflexion of the thumb, whereby the volar accessory (palmar) and the collateral ligaments are torn from their metacarpal attachments, the phalanx carrying with it the volar accessory ligament and sesamoid bones. The head of the metacarpal passes forward between the two heads of the short flexor of the thumb, and the tendon of the long flexor slips to the ulnar side. The phalanx passes on to the dorsum of the metacarpal, where it is held erect by the tension of the abductor and adductor muscles. The attitude of the thumb is characteristic. The metacarpal is adducted, its head forming a marked prominence on the front of the thenar eminence, and the phalanges are displaced backwards, the proximal being dorsiflexed and the distal flexed towards the palm. Many explanations of the difficulty so often experienced in reducing this variety of dislocation have been offered, but the consensus of opinion seems to be that it is due to the interposition of the volar accessory ligament and the sesamoid bones between the phalanx and the metacarpal, and that this is most frequently the result of ill-advised efforts at reduction. In some cases the tension of the long flexor tendon may be a factor in preventing reduction, but the "button-holing" by the short flexor is probably of no importance. Reduction is to be effected by flexing and abducting the metacarpal while the phalanx is hyper-extended and pushed down towards the joint and levered over the head of the metacarpal. When this manipulation fails, the volar accessory ligament should be divided longitudinally through a puncture made with a tenotomy knife on the dorsal aspect of the joint, so as to separate the sesamoid bones and permit the passage of the head between them. An open operation is seldom necessary. Dislocation _forward_ is rare. It results from forced flexion of the thumb with abduction, tearing the posterior and medial collateral ligaments. The deformity is characteristic: the rounded head of the metacarpal projecting behind the level of the joint, while the base of the phalanx forms a prominence among the muscles of the thenar eminence. Reduction is easily effected by making traction on the phalanges and carrying out movements of flexion and extension. The deformity, however, is liable to be reproduced unless a retentive apparatus is securely applied. Dislocation of the thumb to one or other side is rare. Dislocations of the _metacarpo-phalangeal joint of the fingers_ may be backward or forward. They are less common than those of the thumb, but present the same general characters. In the backward variety the same difficulty in reduction occurs as is met with in the corresponding dislocation of the thumb, and is to be dealt with on the same lines.