114 PRACTICAL ANATOMY. 



surface presents a deep, triangular depression for the 

 accommodation of the olecranon process of the ulna; it 

 is separated from the coronoid depression by a thin, 

 translucent partition-plate of bone. 



The lower end of the humerus presents the articular 

 surfaces for the radius and ulna; the outer is the radial 

 head; it is rounded anteriorly and below for articulation 

 with the head of the radius; the inner articular surface 

 is saddle-like, and curves around the inferior extremity 

 of the humerus from the coronoid to the olecranon fossa; 

 it articulates with the greater sigmoid cavity of the ulna. 



The humerus develops by eight centres, one for the 

 head, one for each tuberosity, one for the shaft, one for 

 the radial head, one for the articular surface for the ulna, 

 and one for each condyle. 



THE ULNA. 



The ulna is the inner bone of the forearm. It pre- 

 sents for examination an upper and lower extremity and 

 a shaft. The upper extremity is the larger, and articu- 

 lates with the trochlcar surface of the humerus. It pre- 

 sents the olecranon process, a large, cuboidal process of 

 bone which projects upward from the shaft. On its 

 anterior aspect is a deep, hemispherical cavity, the 

 greater sigmoid, for articulation with the humerus. 

 This cavity terminates below in the coronoid process, and 

 firmly grasps the trochlear surface of the humerus. At 

 the side of the greater sigmoid cavity is a slightly con- 

 cave articular surface, the lesser sigmoid cavity, for 

 articulation with the head of the radius. The coronoid 

 process has a strong base, and contributes in a great 

 measure to the firmness of the elbow-joint. Attached to 

 it is the tendon of the brachialis anticus muscle. The 

 shaft is prismoid in form, and presents three borders 



