Il SKELETONS OF BIRD AND REPTILE 19 
skin formation, and no more a toe than a caterpillar’s 
hind “leg” 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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