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-arm 

 bone (HU, fig. i). 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 pneaxial 

 in position — i.e., 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, 

 praeaxial and postaxial, should be thoroughly under- 

 stood, because, in all the transformations which they 



