4 A BOOK OF BIRDS 



It is owing to the fact that these peculiarities are not generally known, 

 that most writers of books on birds inaccurately speak of the " legs 

 and toes" of a bird, when by the "legs" they refer really only to 

 the long " cannon-bones," which are clad in scales and often brightly 

 coloured. 



A reference to the accompanying diagram should make this clear. 

 Here we have the leg of a fowl. The first joint is formed by the 

 hinging of the femur, or thigh-bone, with the tibia, thus forming the 

 *' knee-joint." At the end of this tibia the joint with the ankle- 

 bone takes place ; but in the birds, as we have said, the two rows 

 which these small bones make up disappear before adult life is 

 reached. One row, as may be seen in fig. i, is com.posed, in the 

 half-grown bird, of a mallet-like piece, the handle of which runs up 

 the front of the tibia or shank, and becomes welded to the " shank " ; 

 while the other (fig. i), originally of several small pieces composed in 

 the embryo, but now forming a thin plate, becomes welded on to the 

 top of the bones of the foot. It is these last which are always, but 

 wrongly, described as the ** leg-bones." Really, they answer to the 

 bones of our own feet which lie between the ankle and the 

 toes. The tibia, and the fused row of ankle-bones, is known as the 

 " tibio -tarsus." The toes present peculiarities, which will be described 

 as we proceed. 



The great lengthening of the foot-bones has been brought about 

 by the evolution of the bird from a climbing to a walking animal. 

 Originally they were five in number, but are now, like the toes, reduced 

 to four. Of these four, three are now welded together — Nos. II. III. 

 IV. — to form a single ** cannon-bone," answering to that of the 

 horse ; but in the young bird their originally separate condition 

 can still be traced (see fig. i). The first of these foot-bones answer- 

 ing to the base of the hind-toe is now reduced to a mere " button " 

 slung by ligaments on to the " cannon-bone." The leg of the 

 old giant reptiles known as the Dinosaurs corresponds marvellously 

 well with that of a modern bird, only in the reptiles the foot- 

 bones had not become welded to form a cannon-bone. So much, 

 then, for the evidence from the skeleton, for the present. The arrange- 

 ment of the blood-vessels, the structure of the eye, ear, organ of 

 smell, and brain, are all on the reptile plan, and so also are the organs 



