PHYSIOLOGY FOR BEGINNERS 



ulna. When the palm of the hand is forwards or looks 

 upwards, the two bones of the forearm lie parallel to each 

 other, the radius on the outer side of the ulna. This is 

 the position of supination. When the hand is turned so as 

 to bring the thumb inwards and the back of the hand for- 

 wards or looking upwards, the radius rotates lengthwise, 

 and its lower end moves across the front to the other side 

 of the lower end of the ulna, describing a complete 



half-circle. In this position, 

 which is called that of prona- 

 tion, the radius lies obliquely 

 across the front of the ulna 

 from the outer side at the elbow 

 to the inner side at the wrist. 

 In the movement of pronation, 

 the upper end of the radius 

 does not move its position, but 

 merely turns on its axis, the cup 

 at the end of the head turning 

 on the humerus, and the edge 

 of the head gliding on the side 

 of the ulna ; while the lower end 

 of the radius, as the bone rotates, 

 travels round the lower end of 

 the ulna, which remains fixed 

 at its pivot. The lower articu- 

 lation, then, between the radius 

 and the ulna is a pivot joint, 

 the ulna being the pivot, and 



-^ the upper articulation is a 



FIG. 2i.-The bones of the right forearm peculiar pivot joint, the radius 



in supination and pronat.cn. rotating on j tselfj the axis of 



A, Supination; B pronation;//, humerus; ^ R . ; 



fi, radius; U, ulna. 



pivot. 



Joints of the Wrist and Hand. The movements at 

 the wrist joint take place between the radius and the first row 

 of carpal bones, between the carpal bones themselves, and 

 between the second row of carpal bones and the metacarpal 

 bones. Numerous ligaments keep the bones in place. The 

 wrist chiefly acts as a double-hinge joint, allowing the hand 



