38 HOUSE, GARDEN, AND FIELD 



out a small paper hand, and attach it to one of the pencils 

 at the end remote from the cork. This pencil will now 

 represent the radius ; the thumb must of course be turned 

 away from the ulna. Mark one side of that end of the 

 radius which is farthest from the paper hand. The paper 

 hand might now be brought round by making the radius 

 simply revolve on its own axis, but there are many muscles, 

 tendons and fibrous bands which pass from the radius to 

 the ulna, and these effectually prevent any such motion. 

 That end of the radius which carries the hand can how- 

 ever easily be made to revolve about the ulna, and this 

 without disturbing seriously the fibres which connect the 

 two bones. The hand can be turned over, the thumb 

 swinging round from right to left, or from left to right, 

 as the case may be. The radius will thereby be crossed 

 upon the ulna, and by looking at the marked end you 

 will see that it has revolved through 180. It is still 

 simpler to take a strip of paper, say 12 in. by 3 in., fashion 

 a rude hand with a thumb at one end, and towards the 

 other end cut the strip along its middle for half its length. 

 Hold the ulnar strip down upon the table and throw the 

 hand of the model over, when the elbow-end will be 

 found to have revolved as before. The radius and ulna 

 are not displaced so far as to strain the bands which pass 

 from one bone to the other, the wrist- joint is undisturbed, 

 and the elbow- joint, mainly formed by the humerus and 

 the ulna, is very little affected. If you can get an actual 

 human radius you will see that the revolution of its upper 

 end is greatly facilitated by two features. There is a 

 smooth, cupped end, which abuts upon a rounded pro- 

 minence occupying nearly half of the end of the humerus ; 

 there is also a smooth circular rim, which rotates within 

 a groove upon the ulna. 



The forearm and hand of man can be changed at 

 pleasure from the supine position (radius and ulna parallel, 

 palm upwards, thumb outwards) to the prone position 

 (radius crossed upon ulna, palm downwards, thumb in- 



