308 BONES OF THE WING. 



much stiffer and stronger than a straight stick of twice 

 the thickness. A \ery portable bridge is often made 

 upon this principle, of a thin plank, bent into the 

 form of a bow by a cord tied to the two ends ; and 

 when tied in this manner, a plank which a man can 

 carry without fatigue, will enable him to cross with 

 safety a chasm ten or twelve feet wide. 



But the bow and the bridge are very imperfect 

 pieces of mechanism, as compared with this portion of 

 the wing. They are strong in one direction only; 

 that is, when the strain or pressure is upon the out- 

 side of the bend, and this is in them the position of 

 least stability. If its position is reversed, the bow 

 bridge is weaker than a straight plank by the whole 

 extent to which it is bent, as the string otters no 

 resistance to further bending. But it will be seen, 

 on looking back at the figure, that the radius, which 

 answers to the bow, is one of very curious curvature. 

 Though less in diameter at the middle than toward 

 the ends, it has its stiffness there greatly increased by 

 being not only straight, but a very little bent the other 

 way. This against a single strain would give it the 

 same strength as a bow has by being thicker, and con- 

 sequently bending less toward the middle than toward 

 the ends. But the bone has to bear strains and twists 

 in all directions ; and therefore, though the double 

 curvature cannot be shown in a figure, it will be 

 found, upon examination of the bone itself, that some 

 part of it is a bow in what direction soever it is turned. 

 The ulna is also both a tie and a strut, and resists 

 equally the drawing asunder and the pressing together 

 of the ends of the larger bone. It is nearly straight, 

 which is the form of greatest strength against both of 

 these ; and it is spindle-shaped, or thickened toward 

 the middle of its length, which is the form of greatest 



