THOEACIC LIMBS 567 



surface of the olecranon, is quadrate, and twice as long as wide. It 

 is slightly wider in front than behind, and at about its middle it is 

 impressed by a transverse crescentic line. The region anterior to this 

 line is quadrate and deeply concave from side to side. The anterior 

 margin is depressed and emarginate; the lateral margins are prominent 

 and rounded. The internal humeral part of the triceps is attached to 

 the anterior part of the inner margin, and the anconeus to the anterior 

 part of the outer margin. The crescentic line which forms the poste- 

 rior boundary of this excavation affords attachment to the posterior 

 humeral part of the triceps. This excavated anterior part of the 

 proximal surface of the ulna is occupied by a mass of fat, forming a 

 cushion, over which passes the tendon of the triceps muscle. Behind 

 the crescentic groove the proximal surface is swollen and convex, and 

 passes without a distinct margin into the posterior surface of the upper 



FIG. 439. 



Coronoid Process. 

 Lesser Sigmoid Cavity jjp^ Greater Sigmaid Cavity. 



ANCONEUS. -nmf^v 'jjjtti -+ Internal Humeral Head of TRICEPS. 

 Lower Lip of Lesser Sigmoid Cavity. ^Hi '^ff^J 

 Posterior Humeral Head of TRICEPS. HB^-.-.-^jy I 



^jifi Jr Scapular Head of TRICEPS. 

 THE ULNA, PROXIMAL SURFACE OF OLECRANON PROCESS. 



extremity. It forms the prominent part of the elbow, and to it is 

 attached the tendon of insertion of the scapular part of the triceps. 



The posterior surface of the upper extremity of the ulna is tri- 

 angular (Fig. 441), having the width of the proximal surface above, 

 and passing below into the posterior border of the shaft. Its lateral 

 margins are distinct ; the inner is sharp and prominent above and 

 rounded below ; the outer is less distinct above and sharper and 

 more prominent below. The outer margin, moreover, is continued 

 downward as the posterior border of the shaft. This posterior surface 

 is smooth and convex from side to side, and from above downward flat 

 or slightly concave in its upper part and gently convex in its lower 

 part. It faces outward and backward. 



The internal surface of the upper extremity (Fig. 442) presents an 

 almost square, slightly concave portion above, and a narrower portion 

 below and behind the sigmoid cavity. This latter region is impressed 



