90 THE FLIGHT OF BIRDS 



rise with ease from the ground, they must be able 

 first to jump into the air. Most birds have an active 

 springing gait, and are good jumpers, their legs being 

 built much in the same way as those of a horse or 

 an antelope ; the ankle-joint is raised high above 

 the ground, and they walk or run upon their toes. 

 The springiness is combined with remarkable light- 

 ness. The bird's foot is made almost entirely of 

 skin, bone, and tendon ; not like the human foot, 

 fleshy and full of nerves. It is worked by long 

 tendons attached to muscles that spring from the 

 top of the leg- bone (the tibio -tarsus) or even from the 

 base of the thigh-bone, and, running in grooves under 

 the ankle-joint, give to the foot its springiness and 

 to the toes their wonderful grip. The fusion of the 

 two rows of ankle-bones with their bigger neighbours 

 makes the ankle-joint an excellent pulley. Alto- 

 gether the bird's wings have in the legs very able 

 assistants. Comparatively few resemble the Swift 

 in having very short and feeble legs ; but, as I have 

 pointed out above, there are a considerable number, 

 mostly big, bulky birds, which, owing to their 

 shortness of leg and small power of jumping, have 

 difficulty in starting to fly from level ground. 



