THOEACIC LIMBS 565 



than the greatest antero-posterior diameter, which is at the proximal 

 extremity. The greatest transverse diameter is also at the proximal 

 extremity, and is a little more than half the greatest antero-posterior 

 diameter. The smallest diameters are the transverse, at the junction 

 of the proximal and middle thirds of the bone, and the antero- 

 posterior, near the distal end. The ulna presents for examination a 

 large proximal or upper extremity, a long, slender shaft, and an 

 unimportant distal or lower extremity. 



The Upper Extremity of the ulna (Fig. 438) comprises the 

 upper fifth of the bone, and embraces the region of its greatest diam- 

 eters. It is a quadrate mass with parallel lateral surfaces and parallel 

 anterior and posterior surfaces and a proximal surface which is in the 

 main level. Its transverse diameter is but half its antero-posterior 

 diameter. The upper extremity is bent inward, somewhat away from 

 the long axis of the bone. It is deeply excavated in front by the 

 greater sigmoid l cavity, w r hereof the lowest point is on the imaginary 

 transverse line which forms the artificial lower boundary of the ex- 

 tremity. The region proximal to the middle of the sigmoid cavity is 

 known as the olecranon 2 process of the ulna. 



The anterior surface of the upper extremity is divided into two 

 parts, the upper and smaller of which forms also the anterior surface 

 of the olecranon process, and the lower and larger part is occupied by 

 the sigmoid cavity. The upper part is twice as long as it is wide, and 

 the long axis is directed obliquely to the long axis of the shaft from 

 above outward and downward. Its proximal border, emarginated by 

 the anterior end of the antero-posterior groove on the proximal surface, 

 is thin and sharp on the inner side and rounded and swollen on the 

 outer side. The lateral margins are also emarginate, but to a less 

 degree. The distal or lower margin is arcuate, sharp, and more or less 

 elevated, and forms the upper edge of the sigmoid cavity. The surface 

 itself is gently saddle-shaped, convex from side to side, and concave 

 from above downward. The inner margin forms part of the area of 

 insertion of the inner humeral head of the triceps muscle ; to the 

 outer margin is attached part of the anconeus. The proximal and 

 distal margins afford attachment to fibres of the capsular ligaments. 



The proximal surface of the ulna (Fig. 439), also the proximal 



1 From its likeness to the Greek letter sigma, which in its old form resembles a C. 



2 From (Gr.) olene, the elbow, and kranion, the head. 



