THORACIC LIMBS 551 



is attached. This is not always well marked. To the outer margin 

 of the supracondyloid foramen, which may be regarded as the lower 

 posterior edge of the internal surface, is fastened the supracondyloid 

 head of the triceps muscle. 



The posterior surface (Fig. 431) lies between the internal and ex- 

 ternal borders. It is concave from above downward by reason of the 

 direction backward of its upper third. Its edges, which are nearly 

 parallel for almost their entire extent, diverge below. The lower third 

 of the posterior surface is divided by a longitudinal ridge into two 

 parts, whereof the inner is principally occupied by the oblong supra- 

 condyloid foramen and the groove above leading into it, and faces out- 

 ward and backward. The outer part, which is at least twice as wide as 

 the inner, faces more nearly directly backward, and is occupied by a 

 triangular depression, shallow at the apex above, but deepening below 

 into the deep cavity on the lower extremity just above the articular 

 surface. This depressed area receives the upper end of the ulna when 

 the forearm is extended, and is known as the posterior supratrochlear 

 or olecranon fossa. The posterior surface above the lower end faces 

 inward and backward, and is convex from side to side, except at the 

 extreme upper end, which is flat or slightly concave transversely. It 

 is smooth and without muscular markings. The following muscles, 

 however, are attached to it : The posterior humeral head of the triceps 

 arises in part from the upper fourth ; below its area of origin, sepa- 

 rated from it by an oblique space, is the area for the origin of the 

 inner humeral head of the triceps. This area has the shape of an 

 inverted shield, with the angle nearest the internal border prolonged 

 downward. The anconeus arises in part from the upper part of the 

 inferior triangular depression already described. 



The Lower Extremity of the humerus differs from the upper 

 extremity in several important particulars. It is compressed from 

 before backward and expanded from side to side, while the upper ex- 

 tremity is compressed laterally and prolonged backward. It articulates 

 by a hinge-joint, moving in an antero-posterior plane with the upper 

 ends of the two bones of the forearm, while the upper extremity 

 articulates by a ball-and-socket joint with one bone only, the scapula. 

 The lower extremity is twisted on the long axis of the bone in such 

 manner that its anterior surface faces outward as well as forward ; 

 the upper extremity is twisted in like manner, but to a less marked 



