VI 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 thé “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 (zz. 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 przaxial 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 (supznus = lying on 
the back); when the palm is downwards, pronation 
(pronus = 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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