THE BONES OF THE UPPER EXTREMITY 87 



groove passing obliquely from within downward and 

 outward between the two portions of this muscle, 

 which is called the musculospiral groove and lodges the 

 musculospiral nerve and superior profunda artery. 



The Lower Extremity. This is divided into an inter- 

 nal and external condyle, an articular surface sub- 

 divided into the trochlea and capitellum. The condyles 

 are rounded eminences of bone extending out beyond 

 the borders of the bone and can be always felt just 

 beneath the skin. Between the two condyles is 

 seen the articular facets, the outer the capitellum 

 is round and smooth and articulates with the cup- 

 shaped surface of the head of the radius; the inner 

 the trochlea presents a deep depression between two 

 well-marked borders and articulates with the sigmoid 

 cavity of the ulna. These articular surfaces mentioned 

 enter into the formation of the elbow-joint, and are 

 held in position by the capsular and internal and exter- 

 nal lateral ligaments of the elbow-joint. Above and 

 in front of the articular surface (trochlea) is a depres- 

 sion called the coronoid fossa, which receives the coro- 

 noid process of the ulna w r hen the arm is flexed; above 

 and behind the trochlear surface is a deep, triangular 

 depression, called the olecranon fossa, which receives 

 the olecranon process of the ulna when the forearm is 

 extended. 



The Bones of the Forearm. The Ulna. This is 

 the internal of the two bones of the forearm. It 

 articulates above with the humerus, externally with 

 the radius, and below the triangular fibrocartilage at 

 the wrist. It presents an upper and a lower extremity 

 and a shaft. 



THE UPPER EXTREMITY. It is divided into an olec- 

 ranon process, a coronoid process, and greater and 

 lesser sigmoid cavities. The olecranon process forms 

 the uppermost part of the ulna. It terminates supe- 

 riorly in front in a peak of bone, which overhangs the 

 greater sigmoid cavity; behind this is a rectangular, 



