178 THE UPPER EXTREMITIES. 



as most of the articulation at the wrist is formed by the 

 radius and the inter-articular fibro-cartilage. The head 

 of the ulna is at the lower extremity of the bone and 

 articulates on the outer side with the radius and below 

 with the triangular fibro-cartilage. From its inner side 

 projects the styloid process. The olecranon process 

 forms the upper extremity and presents anteriorly an 

 articular surface, the greater sigmoid cavity, for articula- 

 tion with the trochlea of the humerus, where it fits into 

 the olecranon fossa during extension. The same articu- 

 lating surface also covers the coronoid process, a smaller 

 projection below and in front of the olecranon, which 

 fits into the coronoid fossa during flexion. Continuous 

 with the greater sigmoid cavity on the outer side is the 

 lesser sigmoid cavity for articulation with the head of the 

 radius. Under the triceps tendon, which is inserted into 

 the olecranon, is a bursa or sac of synovial membrane, 

 such as occurs in parts where much force is brought to 

 bear. 



The Radius. The radius, or spoke of the wheel, is 

 on the outer side of the forearm and gets its name from 

 the way it turns upon the ulna in pronation. The 

 shaft is larger below than above and is slightly curved 

 longitudinally for greater strength. The upper extrem- 

 ity or head is small and has a slightly concave upper 

 surface for articulation with the radial head of the hu- 

 merus. It articulates by its sides with the lesser sig- 

 moid cavity and is bound to the ulna by the orbicular 

 ligament, which runs over a smooth articular surface. 

 Below the head is the constriction of the neck with the 

 tuberosity for the biceps tendon to the inner side below. 

 The lower extremity is large and forms the chief part of 

 the wrist-joint, articulating with the semilunar and 

 scaphoid bones of the wrist. From the lower extremity 

 the strong conical styloid process projects externally. 



In the living the olecranon process of the ulna is always 

 felt at the elbow and the posterior border of the ulna 

 forms the prominent ridge down the forearm, leading 



