BONES, MUSCEES, EXERCISE AND REST. 



273 



are not limited to this single line of motion. The shoulder 

 not only bends forward and backward, but upward and down- 

 ward ; and it can move in any direction, even round like the 

 spoke of a wheel. It would seem necessary, then, that there 

 should be many sets of muscles to produce this great variety 

 of motions. The ball and socket joints are supplied with 



FIG. LV. Internal Muscles of the Back, Shoulder, and Hip. 



a, Muscle that lifts the shoulder- 

 blade. 



6, Raises the arm. 



c, Rolls the arm outward. 



d, e, Rhomboid muscles, draw 

 the shoulder-blade upward. 



g, ' Serrafus, attached to the 

 back-bone and the ribs, draws 

 the ribs down. 



h, Straightens the hip joint, 

 and rolls the thigh outward. 



many muscles, (Fig. LV.,) which act individually, succes- 

 sively, or in combination, and produce every variety of 

 motion ; by these we move the arm, the thigh, the wrist, and 

 the thumb in any direction which we may desire. 



