THOEACIC LIMBS 569 



by a shallow longitudinal groove, from which arise the upper fibres of 

 the ulnar head of the flexor profundus digitorum muscle. The inner 

 humeral head of the triceps muscle is inserted on the outer margin 

 and the contiguous region of the surface, and the supracondyloid head 

 of the same muscle is inserted close to the proximal and prominent 

 posterior margins. Just anterior to this last area of insertion and 

 curving for a distance downward on the inside of the posterior margin is 

 the area of origin of the ulnar head of the flexor carpi ulnaris muscle. 

 A small roughened area behind the lower part of the sigmoid cavity 

 marks the point of attachment of the lateral ligament of the elbow-joint. 



The external or radial surface (Fig. 440) of the upper extremity 

 has the same shape as the internal surface. Its upper part is slightly 

 excavated, and behind is somewhat encroached upon by the posterior 

 surface. Its lowest part is deeply concave, the concavity beginning 

 opposite the middle of the sigmoid cavity and flattening out on the 

 shaft below. The anconeus muscle is inserted on most of the upper 

 part, and the scapular and outer humeral head of the triceps along the 

 posterior margin. The upper part of the extensor indicis springs from 

 the groove on the lower part. On the anterior edge of this groove, 

 a little below the middle of the sigmoid cavity, is a small point of 

 origin for fibres of the extensor carpi ulnaris muscle. 



The greater sigmoid cavity is a deep semicircular emargination 

 on the anterior surface of the upper extremity of the ulna. It is twice 

 as long as wide, and narrower above than below. Its long axis is 

 directed downward and inward, and it therefore forms a small angle 

 with the long axis of the upper extremity, which runs downward and 

 outward, and is also somewhat oblique to the long axis of the bone 

 itself. The upper margin of the cavity is arcuate and sharp. The 

 lower margin is irregular ; its outer half is occupied by the crescentic 

 articular cavity for the head of the radius known as the lesser sigmoid 

 cavity. On the outer side of this cavity the margin is produced into 

 a pyramidal coronoid process. The greater sigmoid cavity is convex 

 from side to side, and is distinctly marked off by the smoothness and 

 density of its surface from the lateral surfaces of the bone itself. It is 

 divided in the middle by the transverse band, more or less broad, 

 which marks the lower boundary of the olecranon and separates the 

 superior and inferior articular surfaces, which are covered with cartilage 

 and move upon the trochlea of the humerus. 



