THE SKELETAL SYSTEM 31 



The humerus is the bony constituent of the upper arm. 

 Its proximal end articulates with the scapula; it articu- 

 lates distally with the radius and ulna of the fore arm. 

 Between these two extremities is the shaft, or body, of the 

 bone. The head is a prominent oval enlargement at the 

 proximal end, which fits into the glenoid cavity, furnishing 

 the articulation with the scapula. The lesser tuberosity 

 is a low knob located immediately in front of the head on 

 the medial surface of the bone. The greater tuberosity 

 is a dorsoventrally elongated projection on the cranial 

 surface of the humerus, immediately in front of the head, 

 extending also on to the lateral surface of the bone. The 

 intertubercidar groove separates the two tuberosities. The 

 deltoid tuberosity is a very prominent rough-edged ridge 

 running, on the ventral surface, from the greater tuberos- 

 ity about half way to the distal end of the humerus. 



The articular surface at the distal end of the bone is 

 saddle-shaped to receive the corresponding surfaces of the 

 radius and ulna. This end of the bone is flattened. It 

 bears two depressions immediately above the articular sur- 

 face — the radial fossa on the ventral side to receive the 

 radius during flexion of the arm, and the olecranon fossa 

 on the dorsal side to accomodate the olecranon process dur- 

 ing extension. The medial epicondyle is a prominent cylin- 

 droid process medial to the saddle-shaped articular surface. 

 The lateral epicondyle is a broad process located laterally 

 near the distal end of the bone. 



The radius and ulna support the fore arm, the former 

 lying anterior to the latter. The two are firmly bound 

 together by the interosseus ligament. The body of the 

 radius bows forward and outward. Dorsally it expands 

 abruptly, forming the head, which bears an oval surface 

 for articulation with the humerus. The posterior side of 

 the head is flattened, and rests against a similar surface 



