CHAPTER II 
THE SKELETONS OF BIRD AND REPTILE 
The Fore Limb 
I WILL now put the skeletons of a bird and of a 
lizard side by side and compare them. Everywhere 
there are striking contrasts, but since the power of 
flight is the prominent: characteristic of birds, or of 
most of them, the fore limb shall be the starting- 
point. At the outset it will be necessary to learn the 
names of the bones that compose it, taking them first 
in the lizard, since there they are more easily made 
out. The first bone is the Humerus, or upper-arim 
bone (HU, fig. 1). Then follow two which lie side by 
side, one slender and one stout. The former of these 
is the Radius (R), and is described as being preaxial 
in position—ze., in front of the axis, the axis of any 
part of a limb being an imaginary central line drawn 
through it lengthwise; the other is the Ulna (U), 
and is postaxial or behind the axis. These terms, 
preaxial and postaxial, should be thoroughly under- 
stood, because, in all the transformations which they 
