THE FRAMEWORK OF THE BODY 35 



girdles and their appendages are built upon the same fundamental plan. 

 There are, however, considerable differences in the sizes and shapes of the 

 corresponding bones. The pelvic girdle is rigidly fused to the sacrum of the 

 axial skeleton, while the pectoral girdle is very loosely and flexibly joined 

 to the thoracic portion of the axial skeleton by means of ligaments, 

 cartilages, and muscles. 



The pectoral girdle and the arm. A pair of slender, curved, rodlike 

 clavicles (collarbones) extends from the upper end of the sternum to the 

 upper part of the shoulder. There each clavicle forms a movable joint 

 with one of the scapulae (shoulder blades). These are broad, flattened 

 plates which extend from the shoulders toward the backbone above the 

 upper thoracic ribs. On the dorsal surface of each scapula there is a 

 flattened, projecting ridge, the spinous process. The apex of this process 

 overhangs the shoulder joint, forms the point of the shoulder, and has 

 the clavicle attached to its inner face. Just inside of the shoulder joint 

 there is another projection of the scapula, the coracoid process. This is 

 the reduced remnant of another bone, fused to the scapula; it forms the 

 point of attachment for certain arm and shoulder muscles. The humerus, 

 or bone of the upper arm, has a rounded head which fits into a shallow 

 socket in the outer end of the shoulder blade; this socket is the glenoid 

 fossa. The ball-and-socket joint thus formed permits great freedom of 

 movement of the arm. 



The skeleton of the arm consists of the humerus, just mentioned, two 

 long bones of the forearm (radius and ulna), and the bones of the wrist 

 and hand. The humerus forms a hinge joint at the elbow with the radius 

 and ulna. The lower end of the ulna lies on the little finger side of the 

 forearm, and its upper end encloses part of the end of the humerus to 

 form the elbow. The radius, at its lower end, lies on the thumb side of the 

 forearm. Its upper end is somewhat cupped, forming a rotatable junction 

 with a convexity at the end of the humerus. This permits the radius to 

 turn so as to move across and partly around the ulna in the familiar 

 movement of turning the wrist (Fig. 3.6). 



The skeleton of the hand comprises eight small wrist, or carpal, bones, 

 five long metacarpal bones imbedded in the palm of the hand, and the 

 phalanges, or finger bones. There are two phalanges in the thumb and 

 three in each of the other fingers. The human hand is essentially general- 

 ized in structure. This is another way of saying that it is not highly 

 modified for the performance of a special function (c/. whales, horses, 

 moles, bats) but is more like the primitive ancestral condition. Never- 

 theless, certain features of the hand and arm may be regarded as 

 specializations for variety of movement and versatility of use. Such are 

 the rotatability of the radius, the great flexibility of the fingers, and the 

 opposability of the thumb to the other fingers. The general usefulness 



