THE SKELETAL SYSTEM 249 



The Humerus. The humerus, which forms the single skeletal element 

 of the upper arm, is the longest and largest of the bones of the upper 

 extremity. It articulates in a ball-and-socket joint with the scapula and 

 in a hinge joint with the radius and the ulna of the forearm. Like other 

 long bones, the humerus consists of a shaft and two extremities. The 

 upper or proximal extremity has a hemispherical head and two tuberosities, 

 greater and lesser, of which the lesser, although smaller, is the more 

 prominent. Shoulder muscles are inserted on these tuberosities. 



The distal end of the humerus is somewhat flattened and divided by a 

 ridge into a trochlea and a capitulimi. The trochlea provides the artic- 

 ulating surface for the ulna while the capitulum articulates with the 

 radius. On the dorsal surface just above the trochlea is a deep pit, the 

 olecranon fossa, which receives the olecranon process of the ulna when 

 the arm is extended. Similar depressions on the ventral surface receive 

 the coronoid process of the ulna and the head of the radius when the 

 forearm is completely flexed. 



Radius and Ulna. The skeletal elements of the forearm are the radius 

 and the ulna. The radius is lateral in position and smaller, the ulna 

 median and larger. The radius is smaller at the elbow and larger at the 

 wrist, the ulna is larger at the elbow and smaller at the wrist. 



The proximal end of the radius is the head. It articulates both with 

 the capitulum of the humerus and with the radial notch of the ulna. The 

 head of the radius is connected with the shaft by the "neck." Between 

 the two, projects the tuberosity for the insertion of the biceps muscle. 

 Distally the radius articulates with the navicular and lunate wrist bones 

 and with the extremity of the ulna. From the extremities of both the 

 radius and ulna project styloid processes which afford attachment for 

 wrist ligaments. 



The proximal end of the ulna is distinguished by the olecranon process 

 to which the tendon of the triceps muscle is attached. By means of a 

 concave semilimar notch the ulna articulates with the trochlea of the 

 humerus. The lower border of this notch is formed by the coronoid 

 process. An adjacent fossa, the radial notch, serves for articulation with 

 the radius. Radius and ulna are so related that the palm may be turned 

 either up or down. In supination of the hand, the palm is up, and the 

 two bones are parallel. In pronation of the hand, the palm is down, the 

 radius crosses the ulna, and the position of their distal ends is reversed. 



The Carpus. The wrist has eight bones arranged in two rows, a 

 proximal and a distal. Beginning on the radial side of the wrist, the four 

 proximal carpals are the navicular, lunate, triquetrum, and pisiform. The 

 four distal carpals are the greater multangular, lesser multangular, 

 capitate, and hamate. Except the pisiform, which is inside a tendon, 

 each of the carpals has six surfaces. Articulating surfaces with the bones 



