ii SKELETONS OF BIRD AND REPTILE 19 



skin formation, and no more a toe them a caterpillar's 

 hind " lei; " is a leg. 



The whole limb is very different from a lizard's ; it 

 is longer and stronger and fitted for an upright car- 

 riage. The strong Tibia is well able to do its own 

 work as well as that of the Fibula, which has almost 

 disappeared. The bird stands on his toes, which are 

 strong and springy, and jumps lightly into the air in 

 order to start his flight. Length of leg is to many 

 species of vital importance, and the elongation has been 

 to a great extent effected by the large development 

 of the metatarsals. The fusion of the two rows of 

 ankle-bones with the longer bones above and below 

 was, I think, necessary for the effective working of 

 the machinery by which a bird is enabled, even during 

 sleep, to grip his perch firmly (see p. 166). Moreover, 

 without all this fusion of bones, would the luxury of 

 standing on one leg be a possibility for him ? 



The horse's leg presents remarkable points of 

 resemblance to the bird's. In both, the Tibia relieves 

 the Fibula of all its work. In both the Femur is 

 short, so that the knee-joint is high and easily remains 

 unnoticed ; in the bird it is hidden among the feathers. 

 In both the ankle-joint is raised high above the 

 ground. 



The Skull 



In a way, the skull is the most bird-like part of the 

 whole skeleton. It is light, not only because of the 

 thinness of its walls, but because of its many air- 

 cavities. Even birds which have their long bones 

 solid have the skull pneumatic. Lightness is of the 



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