The third metacarpal (3) is the longest, and its base is rhomboidal with a projecting dorsoradial angle separated by a groove from the rhomboid surface. The proximal end articulates with the os magnum (_f_) and second metacarpal; the radial surface of the proximal end with the second, and the ulnar surface with the fourth, metacarpal. The fourth metacarpal (4) has a base similar in form to that of the third, and when placed in position with the fifth the two form a hemispherical facet which articulates with the unciform (_g_) and os magnum (_f_). The fourth metacarpal articulates by its radial side with the third, and by its ulnar side with the fifth. The fifth metacarpal (5) articulates by its proximal end with the unciform (_g_), and by the radial side of its proximal end with the fourth metacarpal. _Digits_ (Fig. 51).--The first digit of the hand is called the =pollex= (thumb), the second the =index=, the third the =medius=, the fourth the =annularis=, the fifth the =minimus=. The first digit has two phalanges, each of the others three phalanges. _Phalanges_ (Fig. 51, _i_, _j_, _k_).--The phalanges of the proximal row (_i_) are elongated, flattened dorsoventrally and curved so as to be longitudinally convex dorsad. All have thickened ends. The proximal end is notched, and its proximal surface looks dorsad and is concave for the head of the metacarpal. The distal end is pulley-shaped, and the pulley surface extends farther on to the ventral than on to the dorsal surface, and serves for articulation with the middle phalanx. The phalanges of the middle row (_j_) are like those of the proximal row, but shorter. The proximal surface is triangular and marked by a median facetted ridge. The whole surface is smooth and adapted to the distal end of the phalanx of the first row. The distal end is transversely elongated, so as to be cylindrical, and projects more toward the ulnar than toward the radial side. The distal phalanx (_k_) articulates with this cylinder so that when it is fully extended it lies on the ulnar side of the middle phalanx. The distal phalanx (_k_) has the form of a quadrangular prism. It is excavated on its proximal surface for articulation with the middle phalanx. Its distal surface presents dorsad a deep excavation from the bottom of which arises a compressed plate of bone having the form of a bird’s beak. The depression receives the base of a claw, and the beak-like projection supports the claw. _Sesamoid Bones of the Hand._ _Ossa sesamoidea._--The hand contains, in addition to those already described, eleven small bones that are developed in tendons. One of these, the =radial= sesamoid (Fig. 51, _h_), is closely applied to the radial end of the scapholunar bone. It is developed in the tendon of the extensor brevis pollicis muscle. The other ten occur in pairs as small flattened curved bones on the ventral side of the joint between each metacarpal and the phalanx with which it articulates.