THE BONES OF THE UPPER EXTREMITY 89 



and smooth, and forms the upper part of the greater 

 sigmoid cavity. The inferior surface is smooth and 

 attached to the shaft. The greater sigmoid cavity 

 articulates with the trochlear surface of the humerus. 

 The coronoid process is less marked than the olecranon 

 process; it is smooth, concave, and forms the base of 

 the greater sigmoid cavity. 



The lesser sigmoid cavity is seen at the outer margin 

 of the greater cavity, with which it is continuous; it 

 is concave from before backward; and articulates with 

 the head of the radius held in position by the orbicular 

 ligament. 



THE SHAFT. The shaft tapers from above, is three- 

 sided in its upper three-fourths, slender and cylindrical 

 in its lower fourth. It presents anterior, posterior, 

 and internal surfaces, and anterior, posterior, and 

 external borders. 



THE LOWER EXTREMITY. This presents a rounded 

 head; from its inner and back part the styloid 

 process projects downward, giving attachments to 

 the internal lateral ligament and the triangular fibro- 

 cartilage of the wrist-joint. The head of the lower 

 extremity has an inferior articular surface, upon which 

 the triangular fibrocartilage plays, and an outer, nar- 

 row one, convex for the sigmoid cavity of the radius. 

 The styloid process of the ulna can always be felt 

 beneath the skin. The inner border of the ulna has 

 attached to its length the interosseous membrane, a 

 ligamentous septum stretching to the radius. 



The Radius. It lies to the outer side of the forearm 

 alongside of the ulna. It is a long bone, and articulates 

 above with the capitellum of the humerus, the ulna, 

 internally; the scaphoid and semilunar bones of the 

 wrist, inferiorly. It presents for examination a shaft 

 and an upper and lower extremity. 



THE UPPER EXTREMITY OR HEAD. It is disk- 

 shaped, convex in circumference, and its upper surface 

 has a depression for the capitellum of the humerus, 



