vii FLIGHT 195 



a universal joint. It is at this point only that the wing 

 turns and twists, when it is fully extended : chiefly at 

 this point, when it is partly flexed. The other joints 

 are hinges : had they anything of the " universal " 

 joint in them, the wing would not have that stiffness 

 which is indispensable, if it is to stand the pressure 

 upon it. When the wing flexes some free play in other 

 directions is possible. It will be interesting now to 

 compare the elbow joints of men and birds. In man 

 as in birds the ulna {i.e. the hinder or postaxial one of 

 the two armbones) articulates with the humerus by a 

 simple hinge joint. The difference lies in the articu- 

 lation of the radius, the praeaxial bone. In man 

 this moves with considerable freedom, articulating, 

 as it does, with the humerus by a pivot joint. If 

 you lay your elbow, forearm, and the back of your hand 

 upon a table, you can, while still keeping the elbow 

 immovable, turn the arm so that the palm of the hand 

 faces downwards. The radius revolves upon its pivot 

 joint and the hand with it. In the last-mentioned 

 position it lies across the ulna. It will be useful to 

 remember that the thumb continues the line of the 

 radius. When the back of the hand is downwards, 

 the position is called supination {supinus = lying on 

 the back) ; when the palm is downwards, pronation 

 {promts = face downwards). In the bird the differences 

 are great. When extended for flight, the whole wing 

 is pronated and the elbow joint is immovably stiff. It 

 is true that if we straighten our arm and then place the 

 elbow on a table, the hand will not turn so readily as 

 it did when the arm was bent at the elbow. Still a 

 great deal of free play remains. In the bird the whole 



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