THORACIC LIMBS. 73 



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 sup- 

 ports the claw. 



Sesamoid Bones of the Hand. Ossa sesavioidea. — 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, //), 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 sur- 

 face 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, 



