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.
JOINTS AND LIGAMENTS OF THE THORACIC LIMBS.
The =shoulder-joint= is an arthrodial or ball-and-socket joint. The bones entering into its formation are the scapula and the humerus.
The =capsular ligament= or =articular capsule= is very ample and allows for extended movement of the humerus. It is attached to the edge of the glenoid fossa of the scapula and passing distad covers the head of the humerus and is inserted at the line of junction of the shaft and the epiphysis which forms the head of the bone. On the lateral side of the ventral surface of the humerus the attachment continues distad about two centimeters along the lateral edge of the bicipital groove. On the medial side the insertion passes over the proximal end of the lesser tuberosity. A strong transverse band passes from the greater tuberosity to the lesser tuberosity and bridges the bicipital groove, converting it into a canal. The lateral and medial parts of the capsule are strengthened by thicker bands of fibres, the more prominent medial one of which passes from the coracoid process of the scapula to the lesser tuberosity. To the capsule are closely united parts of the supraspinatus, infraspinatus, coracobrachialis, and subscapularis muscles. A synovial membrane lines the capsule within and forms a sheath around the biceps tendon, so that the latter does not actually enter the synovial capsule.