BIRDS 



143 



■walk on their toes (digitigrade). As a rule, four of these are present, 

 corresponding to toes 1 to 4 in the mammalian foot. (In what birds 

 is the number of toes less?) As birds stand only upon two limbs, 

 they require a large surface of support, which is obtained by the length 

 and spread of the toes. (Man walks on his soles. We can walk on stilts, 

 but cannot stand still on them.) The innermost digit, the ' great toe ' or 

 hallux, is generally directed backwards (posterior toe) ; the second, third, 

 and fourth form the anterior toes. (In which birds does the position of 

 the toes deviate from this arrangement ? 



Inasmuch as the bird is limited to two limbs in its motions on the 

 ground, the body is carried more or less upright, in order to be kept in 

 equilibrium, and for the same reason the legs are generally perfectly 

 developed (name any exceptions, and explain their reason), and firmly 

 united with the vertebral column. The latter result is due to the fact 

 that the bones of the pelvis are firmly anchylosed with each other, and 

 with the immovable sacrum. (In man also the pelvis is anchylosed with 

 the sacrum. Why ?) 



Another peculiarity of the bird's leg deserves special consideration. 

 Many of our birds spend the whole day hopping about among bushes 

 and in trees, the branches of which they clasp 

 with their toes ; and even their night vest is 

 spent in this sitting position upon branches. 

 It would be impossible for us to maintain our- 

 selves in this position by means of our feet 

 (why?), nor could we, like birds, hold on 

 with our hands to the branches of trees for 

 hours at a time, for the overexertion of the 

 muscles would soon become too painful to 

 endure. Birds, however, are not tired out by 

 these efforts, and can maintain a sure and 

 firm seat on the branches even during a storm. 

 How are they enabled to bear this strain ? If 

 we carefully remove the skin from the leg of a 

 bird, we observe a long tendon passing off 

 from a special extensor muscle (a) attached 

 to the pelvis, over the knee-joint (b), thence 

 along the leg backwards across the heel- 

 joint (c), down the tarso-metatarsus, and finally uniting with the toes at cl. 

 The contraction of the muscle must result in the flexion of the toes. 

 The same result, however, is spontaneously attained without the intervention 

 of the muscle whenever the leg is bent upon the thigh, inasmuch as the 

 tendon is stretched by the flexion. (If we bend the knee, the leg of our 



Leg of a Bum, with the Skin 



REMOVED. 

 The ligament b 1 holds the tendon 

 in its place. The other figures 

 are explained in the text. 



