True, his breastbone (_b_, Fig. 33) is deep and thin instead of flat, and those joints of his backbone which lie between his neck and tail are soldered firmly together, more like those of the tortoise, and he stands only upon two feet. Yet this last difference is merely apparent, for if you look at the bones of his wings you will find that they are, bone for bone, the same as those in the front legs of a lizard, only they have been drawn backwards and upwards so as to work in the air. [Illustration: Fig. 33. Skeleton of a Sparrow (from a specimen). _q_, Quadrate bone, peculiar to reptiles and birds and some _amphibia_; _b_, breastbone; _m_, merrythought or collar bone; _c_, coracoid bone, over which the tendon works to pull up the wing; _p_, ploughshare bone, on which the tail grows. Wing bones--_a_, upper arm; _e_, elbow; _fa_, fore arm; _w_, wrist; _t_, thumb; _ha_, hand. Leg bones--_th_, thigh bone; _k_, knee; _l_, lower part of leg; _h_, heel; _f_, foot. ] There is the upper arm (_a_) answering to the same part of the lizard’s front limb (p. 103); there is the elbow (_e_); then the two bones of the fore-arm (_fa_); then the wrist (_w_), and a long hand (_h_), which has lost almost all trace of separate fingers, except the little thumb (_t_), which carries some feathers of its own, known as the “bastard” wing. Now when the sparrow is resting he draws back his elbow, folds his wrist joint, and brings the whole wing flat to his body. But when he wishes to fly he stretches his arms out and beats the air with them, and as his hand moves over most space, it is there that you will find the longest quill feathers, which stretch right to the tip of his wing; then next to these follow the feathers of his fore-arm, while those of the upper arm are short and close to his body, and over all these are the rows of covering feathers, which make the whole wing thick and compact. Here, then, we have the lizard’s front legs turned into a wonderful flying machine in the bird, and this in _quite a different way_ from the flying lizards which lived long ago, and which had only a piece of membrane to flit with, like bats. And now what has happened to the hind legs, the only ones used as legs by the birds? Look at the sparrow as he clasps the bough with his toes, and you will, perhaps, be puzzled why the first joint of his leg turns back like an elbow and not forward like a knee. Ah! but that joint is his ankle, and the knob behind is his heel (_h_), for the bones of his foot have grown long and leg-like; and he always stands upon his toes, the rest of his foot forming a firm support to hold his body up in the air. Look at the skeleton and you will find his true knee (_k_) up above; and if you go to the Zoological Gardens and watch the Adjutant birds, you will often find them resting their whole foot upon the ground (see Fig. 34), and comical as it looks, it will help to explain the curious foot and leg of a bird.